A hasp is part of a latch or lock. Most hasps are flat metal plates with a hinge, often including a metal loop that a lock fits through.
If you've ever used a padlock, you've also used a hasp (probably without knowing what to call it). The metal plate that opens and closes and is held shut once you insert your padlock? That's a hasp! Hasp is a verb too, meaning "lock by securing a hasp," although it's rarely used today. The origin of this word isn't clear, but we know it's related to the Norse hespa, "fastening."