The start and end atoms, \zs and \ze, allow you to define where your match starts and ends respectively within your Vim search pattern. They’re a way that you can tell Vim “hey, I want you to search for this whole pattern, but I only want you to match this portion”
If you search for something like foo\zsbar\zebaz, your search pattern will only match the bar part of the word in foobarbaz.
This is particularly useful with substitutions — the /baz.*\zsfoo search pattern will match the last instance of foo on a line that has the word baz first, so hopefully you can see how these make complex substitutions quite simple.
Try it in the editor!