Chat is a way to communicate nonverbally (send text messages) with the class. Click the Chat button and the Chat area will open on your screen. (You might use it, for example, if you suddenly lost sound and cannot hear your professor. You could ask on Chat if anyone else is having the same problem. You might also use it to ask questions, much as you would in an in-person class.)
Your professor may enable additional features on Chat for your class.