Firestore, being part of Firebase suite and having its roots in Firebase Realtime Database, is another offering from Google (they acquired Firebase in 2014) that wants to ease app development. It is a document database, but this one prides itself in being for backend as well as mobile & web development. Integration should be seamless, everything should work out of the box and all the problems with "ordinary" databases will be solved for you. :)
This won't be another post that glorifies Firestore. I worked with it for quite some time now. I've seen it being used in other projects as well. In all cases it was more of a burden than a relief, at least in the beginning. I think I know why, but let's dig into it a little bit, shall we?