You know how you can get a job done faster, and have more fun, when you do it as a group? That's teamwork! When a group of people work together, they're a team. And teamwork is the good job that people do when they all work together and help each other.
When you and your friends are good at teamwork, that means you do a good job of sharing the work. It's not just one person doing all the hard stuff: it's everybody together pitching in and getting things done.
Even if it's just you and one other person working together, you can still call it teamwork. You and your sibling might use teamwork to carry something really heavy, or reach something that's way up high, or clean up a huge mess that's too big for one person to deal with.