When you unlock your front door, you open the latch. A latch is a fastener or lock that you open with a key.
A latch can usually be opened with a key or by turning a knob or lifting a hook or bar. You can also use the word latch as a verb, like when your mom reminds you to latch the front and back doors before you leave for school in the morning. In some places, the word "latch-key" is used to mean "key." The root of latch is the Germanic word læccan, "to grasp or seize."