Unison is a fancy word that means doing things together at the same time.
For example, if you and a friend are singing a song together and you both start and stop at the same time, that's unison.
Or, think about a group of birds flying in a V-formation. They all move their wings at the same time, which is unison.
Basically, unison means being in harmony and working together as a team. It's like everyone is following the same beat or rhythm, so things run smoothly and sound great!