When browsing through your own pictures you'll see a star icon that you can use to mark a photo as a favorite. Tapping the star will add the photo to a new album titled "Favorites" -- similar to the favorites function in Apple's Photos app.
Google Photos is also adding a heart icon that lets you like other people's photos shared with you -- kinda like how you can 'like' other people's photos on Facebook or Instagram.
Google says the Heart feature is coming soon and the Favorites feature is rolling out this week. You can check out Google's tweets below to get an idea of how these features work.