You might know this word from video games, where you create an avatar to represent you on screen. An avatar is something that embodies something else.
In Hinduism the different gods can take many different forms, and when they took human forms, the human was their avatar. Eventually, the word avatar came to mean the embodiment not just of a god, but also of any abstract idea. If you have a cool head, you might see yourself as the avatar of reasonableness in a fight. Video game avatars are sort of a reverse of the first meaning––a physical entity (you) form becomes something abstract (a video game guy).