Navigate between branches.
Imagine a tree. Yes, a green tree outside in the forest, a tree with many branches. Now imagine some kids playing in that tree. Each kid has their own section the tree, they're playing together. Git is similar.
Each developer can be working on a different branch of the tree at the same time. At the end of the day, developers can submit their code to be combined into one codebase. This allows teams of developers to be working together on different parts of the app, but still allows for merging all branches together.