The EPROM was very important in the development of microcomputers because it gave the designer a way to load his custom program (like bios) into non-volatile memory. This program would always be ready to run at start up and not need to be loaded each time the computer was turned on.

An EPROM, or Erasable Programmable Read-Only Memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, memory it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits.

(From Wikipedia, the free encyclopedia)

Once programmed, an EPROM can be erased only by exposing it to strong UV ( ultraviolet ) light. That UV light usually has a wavelength of 253.7nm (for optimum erasure time) & belongs to the UVC range of UV light. EPROM’s have no limit to read cycles and can store data for over 20 years. EPROMs are easily recognizable by the transparent fused quartz window in the top of the package, through which the silicon chip can be seen, & which permits ultraviolet light during erasing.