x86 instructions are often big and complicated and hard to decode, so much so that the machine doesn't use them internally in the various buffers and so forth that make up it's out of order engine. Instead, they're translated into a different set of instructions called uOps which only live inside the inner part of the CPU.

Translating them takes energy, and can potentially be the limiting factor in how faster certain programs can be run. That means that recent Intel designs keep a small buffer of uOps that correspond to instructions it's likely to see again quickly. This can help a lot in loops.