The term "Door Lock Actuator" is grandiose and mysterious enough to scare away many a do-it-yourself-er; but never fear! Life is not as complicated as it seems.
Basically, a door Lock actuator is a mechanism that causes your car door to lock and unlock automatically- in a sense, before the invention of automatic actuators, the passenger or driver of the car was actually responsible for the task of actuation!
The word "actuator" is applied in this sense to any mechanism that converts energy to motion. Actuators can derive energy from liquid, air, or electricity, and then convert that energy into motion, usually in the form of rotational motion (a turning motor). In a door lock actuator, the energy is provided by electricity from the car battery. This energy is then driven by a small motor into a series of small gears that convert the rotational motion to linear motion, a moving metal rod with a hook at the end. This metal hook, which is oriented vertically, then actually performs the function of mechanically rotating the lock.
A door lock actuator receives tens of thousands of uses over its lifetime, and unsurprisingly it is a common culprit for failure or malfunction. Fortunately, these actuators are relatively easy and cheap to replace. Door lock actuators for most makes and models run about $20-75, so the investment won't break your bank. If your door lock is broken and you'd like to replace it, try searching the internet for instructions for your particular make and model of car- you'll most probably find exactly what you're looking for.