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.
Some incredibly clever guys worked out how to interpret the light information and convert it into something the computer can "read".
The problem is that the laser lens has to be very carefully aligned in order to work. Over time, dust and usage can cause it to move very slowly out of alignment until it no longer reads reliably, causing huge frustration to you, the user! Also, the opening and closing mechanism for the tray can wear out, causing it to stick. (Especially if you have been using it as a cup holder!)
Back in the old days, when replacing one of these things cost up to $100, we used to adjust the tiny screws that aligned the laser beam in the hope that it would start to read reliably again. (with varying degrees of success). Sometimes just cleaning the dust off with a tiny brush or soft cloth or even a cotton bud dipped in Isopropyl Alcohol was enough. That's still worth trying - but don't use the Isopropyl for medicinal purposes or the main thing that will go out of alignment will be your brain cells...
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!
The main thing here is to get the correct type of drive. The newer drives connect using SATA cables. The power connector has a black plug end and comes from your power supply. The data cable is often red or orange and is quite slender (about 1cm wide). As long as you connect the new drive in the same way as the old one (take a photo when you pull the old one out if you are unsure), your computer will detect the new one when it is restarted. Don't forget to turn off the computer to do the swap!