Usually lasting your first 300 turns, protection keeps you from being attacked while also preventing you from attacking
Protection can be exited early via the Manage screen, but it is not recommended unless you are an experienced player. It is to your advantage to use the turns you have in protection to build up a bit so that you're not defenseless when released from it.
It is also recommended that one comes out of protection with full turns, so go ahead and use 300 turns (assuming you have 301 turns in protection) then wait until you have full turns. This allows you to make a good 'run' and build up a bit before someone has a chance to attack you.