"I could care less" means that you don't care about something at all. Use this phrase to talk about something that annoys you. For example:
I could care less what she has to say. I'm done with her!
To be honest, I could care less about sports.
"I could care less" is actually incorrect. The correct phrase is "I couldn't care less", meaning that it's impossible for you to care about something less than you already do. However, a lot of people say "I could care less" as well. It's technically a mistake, but a lot of people make this mistake.