An Optical Drive....you know, those things that look like a cupholder when they are open. You put in a CD or DVD, it spins up and gets information or plays them on your computer.
They seem pretty reliable. Until they stop working. Or, worst of all, they SOMETIMES work!
These drives work by projecting a laser beam onto the surface of your DVD disk while it spins around. Information comes off the DVD because it is stored in pits and hollows in the plastic (not totally unlike a vinyl record if you remember those...) and the laser light reflects off them in different ways.
These days, as they now cost less than $40, the most economical method is to REPLACE the drive. It's a very simple process in most cases. Take the two sides off your computer case, unscrew the small screws that hold it in to the case (usually on BOTH sides), carefully detach the two cables that connect it to the motherboard (power and data) and slide it out of the case. Some cases need the front panel removed!