What do people mean when they say you could be arrested and charged for something you didn’t do with 100% evidence you didn’t do it and no one believes you?

Like I’ve seen people say the system is messed up with innocent people being wrongfully charged with evidence they didn’t do it. I don’t really understand how that works though. How can you be charged with something you didn’t do, with evidence you didn’t do it, but still be in trouble?