Total depravity is a kind of big word that people use when they want to talk about how people are after doing bad things. It means that all people have sinned or done bad things, and they keep doing them all the time. It does not mean that everyone is as bad as they could be, but it means that everyone is very bad because they have disobeyed God's rules. This idea comes from the Bible which tells us that since Adam and Eve did wrong things, everyone born after them was born with a sinful nature. This Sinful nature makes people do things that are wrong in their hearts, actions and mind.
For example, let's say that you have a cookie, and your mom told you that you can only eat half of it, but you went ahead and ate the whole thing. This is a wrong thing that you did. Now imagine that you also didn't share your toys with your friend when they came over and broke their favorite toy. And you also lied to your teacher when they asked if you did your homework. These things are all different types of wrong things too.
Total depravity means that even if you try really hard not to do wrong things, you will still do it because it is in your nature. However, there is hope because Jesus came to save us from our bad behavior and nature, and make us new again. That means we need to ask Him for forgiveness and try to follow His rules.