Morrowind Rebirth, as you would imagine from the title alone, is a TES: Morrowind mod that is all about enhancing and modernizing the gameplay through a variety of balance changes and new content. It features new creatures to fight, new armor and weapons to collect, massive zone overhauls, various tweaks to the magic system that make it less of a broken mess, and the list goes on for quite a while.
It has been a work in progress for almost a decade now, yet there don't seem to be any signs of the development slowing down. If anything, it would appear that the development is currently going faster than ever given how Morrowind Rebirth has just been updated with the rather hefty 4.2 patch. There are simply far too many details for me to cover in brief, so if you're interested in the specifics you'll find the full patch notes over here.
On the other hand, if you're more interested in giving it a try and seeing how much it has managed to improve Morrowind, you can find the download like over at ModDB. The installation process is not very long, but it is a bit tricky for those without experience, so if you encounter any problems make sure to visit the tutorial. As a final note, here's one of the most recent trailers, just to give you a brief idea of what kind of mod you're getting yourself into. Enjoy!