MPSync is a mod for the game Satisfactory that allows you to Sync your multiplayer saves to the cloud. This allows you and your friends to play the same save file even if the original host is not online.
Whenever you are hosting a "friends only" game and you save. The save is uploaded to a cloud server. When your friends want to play, they can use the MPSync menu to find your cloud save, download it and host it themselves.
There is a help button in the in-game MPSync menu. This has many FAQ answers. Should you need further assistance you can join the Satisfactory Modding Discord Channel and ask there. Either myself or someone there will be happy to help.
There seems to be an issue when you are logged in to Satisfactory with both Epic and Steam. I am yet to come up with a solution for this so it is probably best to log out of Epic if you plan on using Steam. This can be done via the Satisfactory->Online menu.