An expansion bay or bay is an open section on the computer used for expansion for add-ons to your computer, such as a hard drive and CD-ROM drive. Drive bays are available in 3.5-inch and 5.25-inches. The picture is an example of what the front of the computer may look like with an example of an empty 3.5" and 5.25" drive bay.
Before installing a new drive, such as a CD-ROM drive, remove the front bay cover from the front bezel to provide access to the drive. When installing a hard drive, the drive bay cover can remain in place.