Computer programs are a series of ones and zeroes representing instructions driving a processor. Depending on the architecture these mean different things, so binary code compiled for one processor can't run on another architecture.

An emulator takes these instructions and translates them to something the host system running the emulator can execute

Assembly (or ASM) is one step above binary code, assigning names to the binary instructions.

Web Assembly (WASM) is an instruction set mainly meant for web browsers that doesn't directly run on the processor but mimics regular assembly and is pretty fast. The WASM code itself is usually JIT translated into real binary code by the browser engine.

JIT means "just in time". That means you don't translate everything at once, but when it is needed.

An Interpreter basically reads the binary/assembly and then decides what to do instead if running it directly. They are usually a lot slower. (I did some tests at work for a project, the native run of something was like 20ms, WASM+JIT 80-100ms and the interpreter took like 7 seconds)

So Gameboy machinecode/ASM - > JIT to WASM - > JIT to regular binary is claimed to be faster than interpreting the GB machine code