Back in February, Instagram began allowing up to 10 photos/videos in one post. However, they were restricted to a 1:1 (in layman's terms, a square) aspect ratio. With the latest version 12 update for Instagram on both the Play Store and Apple's App Store, those multiple photos/videos can now be in any aspect ratio, so long as the photos/videos within one post are of the same ratio.
So that main aspect of the update is pretty self-explanatory. Whereas you could previously only choose a square ratio for your pictures/videos when you put a few in the same post, they can now be whatever you want. Whether that's 4:3, 16:9, 3:2, or whatever wacky ratio you decide to upload your media in, you can do so, as long as all of the media in that post follows the same ratio.
Moving on, Instagram also updated the design of its app, even if it didn't disclose having done so. Everything is more open and less cramped, with a thinner font. The settings (called "Options" by Instagram), the search results, the account switcher - everything is more spread-out in general. The filters also have the first letter of their names on the blurry preview now, which is kind of useless, but whatever. This seems to be a server-side update, as I'm on version 11 and already have the new visuals.
Overall, this is a solid update. I've had the new stuff for a few days, so version 12 is definitely already hitting the Play Store. If you still haven't received the update, you can grab it from APK Mirror.