In Microsoft Teams favorites allow users to prioritize some teams over others. Teams that are in your favorites list will appear above your other teams. You should favorite teams that are the most relevant and useful to you. In order to view your favorites, click on the arrow to expand the list. (screenshots)
Adding a team to favorites
Click on the More button to expand the list of teams. (screenshot)
Find the team you with to add to favorites and click on the ellipses to expand the menu. (screenshot)
Click on Favorite. (screenshot)