If you have a few favorite hashtags on Instagram that you keep your eye on, the only way to do so until now has been to search for them and see what's newly tagged with them (unless a third-party app had the option to follow them). Now Instagram is giving you the option to follow tags, just like you would people. It's one of the Instagram features that were leaked a few weeks ago.
Either search for a hashtag you're interested in or tap on one from a post in your timeline and you'll see a pop-up telling you that following hashtags is now possible. Every hashtag page also shows related ones so it's easy to jump to similar photos and follow a bunch in one go.
After you follow one, you'll be able see the top posts from it in your timeline and the latest stories tagged with it as well. Followed hashtags will also show up in your profile, if it's public, and other people's as well. So you can get inspiration for hashtags to follow from your friends if you're out of ideas.
Instagram says the feature is rolled out, so it should be live for you in the Android app. It showed up for me on version 18.104.22.168.97, which you can grab from the Play Store or from APK Mirror.