Agender describes a person with no gender identity. It means that someone does not feel like they're any gender. "A-" is a prefix that means 'no' or 'none.' An agender person feels like they're neither a boy nor a girl.
Agender is also called "genderless," "ungendered," and a few other things. It includes a lot of people. Some agender people may identify, or call, themselves transgender. Not all do, though.
Many use "they" or "them" as pronouns. Again, not all do. Agender people are very different from each other sometimes! Some wish to change their bodies to feel happy. Some don't.
Angel Haze is a very well known agender person. Angel is a rapper and uses "they" and "them" pronouns.