We will now be talking about ISO which, surprisingly, is an organization. ISO stands for international standards organization, but its basically a standardized industry scale for measuring the sensitivity to light.
As you can see in the photo below, the ISO is right below the F stop number.
When do you mess with your ISO? Well, ISO is used to increase your sensor's sensitivity to light. The higher the ISO the more light your sensor will take in. So basically you would raise your ISO in low light situations. There is just a few problems when using a high ISO, and the biggest being the grain. When using a high ISO photos will become much grainier depending on how high your ISO is.
This would be a good time to mention that there are two main types of cameras that professional photographers will use. Of course there are other types, but for the most part most photographers you meet will either be using a crop-sensor camera or a full-frame camera. Full frame cameras have a full sized sensor while crop sensor cameras have a 1.5x crop or 1.6x if you are using a canon. All this means is that a crop sensor is 1.5 times smaller than a full frame camera. So why am I talking about this now? Because a full frame camera has much better low light performance. Not only because it has a larger sensor but because it handles higher ISO much better than a crop sensor camera.
Before things get any more confusing lets look at some examples from both a full frame camera and a crop sensor camera at different ISOs. (Not taken by A K Photography)
Here we have photos from the Nikon D7200 DX (Crop Senor) camera and the Nikon D750 FX (Full Frame) camera. At first glance the comparisons are exactly the same, but once you look closer you'll notice that the settings for the D7200 are half of what the D750's are. Not only is the crop sensor grainier at a lower ISO, but you have to compensate by lowering your shutter speed, and a faster shutter speed is always better when you're shooting action in low light like indoor sports or weddings. Here are pictures of a full sized sensor and crop sensor so you can have a visual representation of what these two look like.
Just as a side note. Any lens you put on a crop sensor camera will have to be multiplied by the crop factor. For example, if you put a 50mm lens on a 1.5 crop sensor then the lens will actually be at 75mm or 80mm if you're using canon. So just keep that in mind.