Read about microcode in general and processor pipelines.
Microcode is the program code one level below assembly, that is used to encode the logic of complex instructions. It is not accessible to programmer and virtually everything about it is a trade secret of computer manufacturer.
The processor pipeline allows you to execute multiple microcode instructions at time, if they use different parts of CPU. It is a great performance booster.
This document contains a good insight on these topics: The microarchitecture of Intel, AMD and VIA CPUs: An optimization guide for assembly programmers and compiler makers