26 - Index - is an alphabetical list that you can use to look up the locations of terms. It is an ancient form of information sorting that allowed for easier information retrieval. It operates similar to your memory in the sense that it relies on a retrieval cue in the form of a word.
It is a layer of structure in the zettelkasten that works because it is utilizing a well known model (memorized alphabet) that allows you to narrow your search options. When you go to look up a word (your retrieval cue), you immediately narrow down the options in the index to words that start with your letter and can jump to that section.