This is one of those IPs that seemingly has no beginning or end. As someone unfamiliar with it, I have no clue what's going on with all these numbers. And if that wasn't confusing enough there are multiple distinct games under the same roman numerical, like here.
I don't care for this IP (or the company), I don't play these games, but I understand how the series is set up. Each game with a roman numeral is standalone. They don't have anything to do with the other ones. They are not sequels to each other, they are not even related to each other. So FF7 has nothing to do with FF4 which has nothing to do with FF6 or FF9 or any of the other ones. They are each distinct games.
If one of these games
does receive a direct sequel, like FF10 or FF13, then a second number is added, like FF10-2 or FF13-2.
FF7 Rebirth is the 2nd of 3 parts that together comprise a complete total remake of FF7. Part 1 was FF7 Remake. Part 2 is FF7 Rebirth. And there will be one more coming I believe to conclude this remake trilogy.
Various different FF games have received remasters or remakes over the years.
As for why the series is called "Final Fantasy" to begin with, that's because the very first game was going to be the company's very last game before they went under. However the game saved their @sses, and they eventually became known as a JRPG powerhouse. Boy wasn't that lucky.
Personally I've never cared for any of the FF games. I've tried to play a few of them, like FF4, FF6, FF7, FF8, and FF9. I could only get a few hours in before I said "f*ck this, I'm out". On the other hand, their game Chrono Trigger is one of my favorite games of all time.