So I previously mentioned that ISO is the sensitivity of the camera to light. It originally came from film where you could buy different types of film with different sensitivities to light. You would buy the ISO film you needed for the type of photo you wanted. Thankfully, while in the past the ISO of the film was set for a roll of film, with digital we have the ability to vary the ISO as we shoot which is incredibly helpful.
High ISO means high sensitivity to light. This means you don't have as much light to get the correct exposure as a low ISO which has low sensitivity. The benefit of this is that when you shoot in low light situations, you can still get decently exposed images without having to blow out your shutter speed to something that'll get very shaky and blurred. The disadvantage is that higher ISO also means you get higher amounts of grain in your images. As such, as a general rule of thumb you would shoot with the lowest ISO that you can get away with.
To help you understand the ways in which ISO interacts with the shutter speed and the way in which is can get grainier at higher ISO, here are some example images for you.
ISO 100 F1.8 1/5s
ISO 800 F1.8 1/40s
ISO 3200 F1.8 1/160s
ISO 12800 F1.8 1/640s
ISO 25600 F1.8 1/1250s
You see how the higher ISO images are grainier? That's effectively what the downside is of shooting at a high ISO. Of course, sometimes it's just necessary such as when you're shooting in low light and you can't open your aperture up anymore or decrease your shutter speed without causing some unwanted blur. I emphasise unwanted because in some instances you actually want it to blur.
If you look closer at the ISO and shutter speed values, did you notice that while I had fixed the aperture, the shutter speed kept increasing as the ISO went up. This is occurring because as the camera is more sensitive to light, it needs less light to get the right exposure. So it does this because it's cutting out more and more light using the faster shutter speed and compensating with an increased ISO. Note that as the ISO doubles, it is equivalent to one stop of light. This means you can halve your shutter speed.
Now the amount of grain can vary depending on your camera. Cameras with larger sensor sizes tend to have less grain at the same ISO than a camera with a smaller sensor size. This is why people quite like full frame cameras because they have a large sensor and so the level of grain at a higher ISO is significantly lower than for say a crop sensor camera.
Now yes you can see that at extremely high ISOs it's so grainy you'd question why you'd ever use that. Well, in some cases at night when you're shooting a landscape, it's so dark that you can't even see what's in front of you. If you try to shoot such a scene at ISO 100, you'd spend 10 minutes waiting to see if your focus/composition is even right. So what I do is bump up the ISO like crazy and then in like 10 seconds you can see your composition and check it. Then if you're happy with it you can dial it back to ISO100 and take your shot.
This shot has a shutter speed of 259s. I could not actually see anything when I was shooting so I dialed up the ISO to max, composed the shot, check the focus then went back down to ISO 100 to take the shot
Another term you'll hear people throw around a lot is depth of field. Depth of field refers to how quickly the background behind the subject blurs. For a shallow depth of field, everything behind the subject will blur very quickly whereas for a large depth of field, you'll find items quite far behind the subject aren't blurred at all.
So basically when people want a shallow depth of field, they mean they want to make the background behind their subject to be blurred. Now the way focus works is that a focus plane basically exists in concentric circles all around you. So if you were to focus on something 2 metres away and then turn around in a circle without shifting focus, you would notice that everything else that is two metres away the whole way around you is actually in focus.
The aperture value you dial into your camera, which as you recall is the size of the opening in the lens, will vary your depth of field. So if you have a very large opening, i.e. a low f stop number, then you'll find they produce a very shallow depth of field so everything blurs quickly as you move away from the subject. Conversely, if you have a small aperture, i.e. a high f stop number, then lots of items will still be in focus. So as such you'll find for portraits, people will generally like a high aperture which will give them a shallow depth of field.
You see how the background is really blurry here? That's because I shot with a high aperture lens and chose a high aperture (i.e. a low f-stop number)
Although depth of field is greatly affected by aperture, it's not actually the only thing that affects your depth of field. Other items that can influence your depth of field include:
Distance between your subject and the background - larger distance between them and background gives the look of a shallower depth of field
Distance between you and your subject - the closer you get to your subject, the shallower the depth of field. Be mindful of this when shooting macro
The focal length of the lens you're using - longer focal lengths give shallower depth of fields
This photo has a shallow depth of field despite being taken at f5.6 This is because it's shot using a 250mm lens. If you had used f5.6 as 18mm, you not get anywhere near as shallow depth of field.
A shallow depth of field is pretty typical for portraits like this one and is usually achieved by shooting with a high aperture i.e. a low f-stop number of around f2.0