Keep the things you use the most easy to find - your favorites appear in the top left of your sidebar, above other channels.
Favoriting is a personal action, so nobody else on your team will see your favorites.
Use the star icon to favorite something ⭐️
You can click on the star icon in the top right of a doc to favorite it.
You can click on the star icon next to entire channels in the left sidebar to add them to favorites, as well.