Okay kiddo, so have you ever heard someone say something nice to someone but it actually sounded like it wasn't nice at all? That's called a backhanded compliment.
It's like saying "You look pretty good today, considering how bad you looked yesterday." Even though the person said something nice, it still feels like they insulted you a little bit because they made a comparison to how you looked before.
Or if someone says "Wow, you're so smart for someone your age!" That doesn't sound too bad at first, but it also kind of implies that being young means you can't be smart.
So basically, a backhanded compliment is when someone tries to say something nice but it sounds like they're actually being a little mean or insulting.