There are a few meanings for "informal". "Informal language" is when you're talking in a relaxed way like how you would talk to your friends or family. The opposite is "formal language" which is how you would talk to your boss or to elders. "Informal dress" is when you wear normal, every-day clothes. "Formal dress", then, is when you wear a fancy dress or a suit to a wedding or something like that. Basically, "informal" means relaxed, friendly, casual, ect. I hope that helps!