An EPROM is an "Erasable Programmable Read Only Memory" chip - basically a Blank ROM where you can "Burn" Programs, Data, and Codes to. A ROM is a "Read Only Memory" chip where all this information is already burned onto the chip.
To get a EPROM copy of a ROM, one has to copy that ROM onto a file and then using a EPROM Programmer, burn the content of the file onto EPROM. This will give you a EPROM copy of the ROM. You can then use the EPROM copy of the ROM as if it were an actual ROM (and in many {many many many!!!} cases it is done this way). All computer manufacturers use EPROMs in one form or another on their systems, including Apple. My Classic II's board being a low serial number makes it either a Prototype or Post Production Prototype Classic II uses 4 EPROMs on its board.
Now for the LEGALEESE...
Because of Copyright issues, it is not (technically) legal to copy ROMs from a system. But people do it anyway for one reason or another. The Copyright is to protect the owner (in this case Apple Inc.) from having its ideas stolen and used on other systems like Cloned systems.
The only loophole around this is making ROM Copies on EPROMs for repairing or restoring original systems when no spare ROMs (and other parts) are available. This is where your case fits in - you got a bad ROM and only a EPROM copy can fix this if you can not get a ROM from your machine.
