Friendica is a sort of Swiss Army knife of the Fediverse: it lets you follow and interact with Mastodon accounts, but it also lets you do a lot of things that other Fediverse server types can’t do. It can be more complicated to use, but if you can get the hang of it it offers features that other server types don’t have.
The website version of Friendica has an interface a bit like Facebook from a few years ago, lets you make long posts with no character limits, and follow accounts from all across the Fediverse including Mastodon etc. As well as posts, Friendica lets you create and share calendars, photo galleries and groups (though the groups work very differently to those on Facebook).
You can also follow RSS feeds, and each RSS post will appear in your home timeline as if it was from a normal account on the Fediverse, so it can be replied to or shared with others (the original RSS feed creator will not know about it though). Additionally, you can turn RSS feeds into specific Fediverse accounts that people on other Fedi server types can follow.
If you’re on the old Diaspora social network, you can use Friendica as a stepping stone to the Fediverse as it has compatibility with both. From Friendica you can follow Fedi accounts on Mastodon etc. and also Diaspora accounts, with everything displayed in one feed.
There is no dedicated Friendica app, but you can use it with most Mastodon apps, and if you have an Android device you can use the Fedilab app ⧉.