Backsliding is when you were doing something good, and then you stopped doing it or started doing the opposite.
For example, if you were learning how to read and you were getting really good at it, but then you stopped practicing and forgot some of the words you learned, that would be backsliding.
Sometimes people backslide in their behavior too. If you were being really nice and kind to your friends, but then you started being mean and not sharing, that would be backsliding.
It's important to try and always do the things that are good for us and for others, even when it gets hard or we feel tempted to stop. Because when we backslide, it can be harder to get back to where we were before.