Refers to a storage locker, such as those found in schools, gyms, or stations. The Japanese usage is essentially the same as the English loanword.
I left my luggage in a locker at the station.
I keep a change of clothes in the gym locker.
Please put your valuables in the locker.
I rented a coin locker at the station and left my luggage there.
Once you've changed, be sure to lock your locker.