An Optical drive is a place where you can insert CD's and DVD's and run them on your computer there is also a quick video down below and it is about how and which optical drive to buy, there is also a video of a optical drive tutorial. also there is a very very detailed picture which explains pretty much every part of an optical drive.
These videos are a bit long and boring but they include a good amount of information in them.
How to Choose an Optical Drive
Optical Drive tutorial
They range from about £10.00 - £20-00 so its not to bad and it's quite easy to install there is a install video underneath this.
How to Install an Optical Drive(DVD/CD Player)
Above this text there is a video of a person explaining an optical drive in detail and he shows you all of the parts that you can get to and he says what they do and what they are called.