Imagine you're playing with your building blocks and you've built a tall tower. Suddenly, your little sister comes along and knocks down the tower, which makes you sad. You wish you could go back in time to before your sister knocked the tower down, so you could start building again.
That's kind of like what a rollback is, but with computer programs instead of building blocks. Sometimes when people are working on a computer program, they make a mistake or something goes wrong, and the program starts acting funny or stops working altogether. This is kind of like your tower falling down.
But, like you wished for with your tower, the people who are working on the program might wish they could go back in time to before the mistake was made, so they could start over and fix whatever was wrong. This is a "rollback."
It's like pressing the rewind button on a movie or video game, and going back to an earlier point in time before something went wrong or before a particular action was taken. Once they've gone back to that earlier point in time, the people working on the program can try to fix whatever's wrong, just like you can start building your tower again once it's been knocked down.
The idea with a rollback is to get the program back to a point where it was working correctly, so that it can be used without any problems. This is kind of like starting over with your building blocks and trying to build the tower higher and stronger than before.