Yeah, normal ordering and contractions, if we're expanding the fields in terms of such operators when doing canonical quantization using Fock spaces. There's a

Yeah, normal ordering and contractions, if we're expanding the fields in terms of such operators when doing canonical quantization using Fock spaces. There's a lot of stuff going on there. The fact that we can draw nice pictures representing the terms of a perturbation theory doesn't mean that you can skip a regular course on QFT to understand what practitioners mean when they use them. Again, not stressing this while showing tree level Feynman diagrams as if they were straight representations of particle interactions may give wrong ideas to the unsuspecting readers.