Can someone explain what exactly open.mp is and what does it do?

did you access the website, at least? https://www.open.mp It's described very clearly here...

@gray-grass-64213 How is it different from SAMP?

did you read the FAQ?

I did

They're explaining pretty well there, but here goes... SA-MP is getting old, and it's a matter of time before the official website goes down (in my opinion) Open.MP is SA-MP's successor. It was created with entirely new code, and it improves a lot of stuff. You can load your gamemode, scripts and plugins in the Open.MP server and they should work just fine. But if you want to recomplile your scripts with the new libraries, you'll have to change some of the code. A lot of functions were slightly changed. The server is already released, and now they'll be working on a new client. There's also the fact that the majority of SA-MPs documentation has slowly disappeared. The forums are gone. Open.MP will continue to have an up-to-date documentation, and an active community to support it.