It's a command.
"Eve geliyor." = He is coming home. Just a normal statement, not a command.
"Eve gel!" = Come home! (to a person you would use "sen" to: Komm nach Hause!)
"Eve gelin!" = Come home! (to a person or multiple people you would use "siz" to: Kommt nach Hause! Kommen Sie nach Hause!)
Entweder höflich ("Kommen Sie!") oder einfach Mehrzahl ("Kommt!").
It's used when you give a command to siz.
So, either to several people, or to one person that you are speaking to politely.
If you give the command to sen, then you don't use the -in suffix, but just the bare verb stem.