I thought locks were just warlocks, but after me and my raid failed normal SoO 2 minutes before server reset, everyone said "atleast we have lock"

Idk what this means???????

lockout as in you can create a group for just killing garrosh for fast heirloom run

If I create a group and we can't kill it and the raid disbands will I still have the lock?

And after I kill garrosh do I have to rerun the entire dungeon to Garrosh again?

You will have the instance lock @ Garrosh, and no you will not have to rerun the entire dungeon to Garrosh again.

You can extend a lockout but you can only do it if you've actually downed immer through klaxxi.

After you've killed it you will need to get a new lock for the next run (After resets on tuesday)