A cavity for a disk drive in a computer cabinet. Drive bays today are typically 5.75" wide, which accommodate 5.25" optical drives and 5.25", 3.5" and 2.5" storage drives. See hard disk
and Device Bay
Half-Height vs. Full-Height
Drive bays today are "half-height" drives that are 1 5/8" high. The half-height designation is because the bays used to be 3.25" high ("full-height") to hold the floppy and hard drives in the first personal computers.
Four Half-Height Drive Bays
This computer has four half-height drive bays, three of which are empty below the DVD drive on top.
CP/M computers in the early 1980s had two full-height drives. The first personal computer hard drives held 5MB. Today's entry-level smartphone has 3,000 times as much storage.