An emulator is a computer program that executes the machine code of some system. For example, if your computer is x86, you can't natively run ARM machine code. But an emulator can.

QEMU is an emulator that can run entire operating systems, because it emulates hardware devices like hard drives and displays. Unicorn doesn't emulate any of those things, it only emulates the CPU. It's probably mostly useful for compiler development and security research / reverse engineering.