This article explains what a restore point is, when Windows creates one, what's in a restore point, and more.

A restore point, sometimes called a system restore point, is the name given to the collection of important Windows system files saved by System Restore at a specific date and time.

What you are doing in System Restore is going back to a saved restore point. If no restore point exists on your computer, System Restore has nothing to go back to, so the tool will not work for you. If you are trying to recover from a major problem, you will need to move on to another troubleshooting step.

The amount of space that restore points can take up is limited (see Restore Point Storage below), so old restore points will be deleted to make room for newer ones when that space is full. This allotted space can shrink even further as your total free space shrinks, which is one reason we recommend keeping 10 percent of your hard drive space free at all times.