System Restore is a Windows function that allows you to "rollback" the system files to the earlier point in time when a "snapshot" is created of each important system file and some application file changes. But note: System Restore is not a System Backup.
The difference: System Restore is taking a snapshot of system files plus a very limited set of other system resources. A (full) System Backup is taking a snapshot of every EVERYTHING.
System Restores are a great idea as they can run quickly but in too many cases become a user's default "System Backup" - until realizing that System Restores give you a chance at recovery but it may not, in fact, have saved EVERYTHING needed to get to that rollback point because many other changes occured since that point which were never saved so can't be restored.
Also note, you have a limited time window to use a System Restore point. Windows reserves a maximum amount of space to save them. Once that space runs out, Windows removes older System Restore points to make room for a new System Restore.
SYSTEM RESTORE IS NOT A SYSTEM BACKUP REPLACEMENT.
RUN SYSTEM BACKUPS AS WELL