The Interest feature is to show your friends what your interests are.
You can use the "Interest feature"
1) by going to the Discover tab, clicking on the # icon at the bottom right corner and selecting one or several of the interests,
2) by going to your profile and selecting your interests.
After you've selected your interests and pressed OK, try the feature by swiping right from the Discover tab!
You can also see your friends' interests by checking their profiles.