If you want to implement a complicated computer you have two choices: you can create a complicated logic circuit that does what you need (this is called "hardwired") or you can create a far simpler computer and program it to emulate the complicated one (this is called "microcode").
https://en.wikipedia.org/wiki/Microcode
Since the simple computer will only ever run one program, the emulator for the complicated computer, it can be very specialized. That makes a microcoded IBM 360 very efficient compared to a Z80 emulating an IBM 360, for example. Normally microcoded machines have a tiny special memory to hold the microcode program, from less than a hundred to a few thousand words in size. The Gigatron has its "microcode" in the EPROM while the emulated code is in the RAM.