In this technique, what exactly is a locus?

A single, specific spot along your route where you park one image — a station in the journey.

Loci (plural of locus, Latin for "place") are the pegs of the palace: your front door, the hall table, the kitchen sink, each in a fixed order. A locus should be distinct and stable — a particular object or position you can picture unambiguously — so that when you "arrive" there on recall, exactly one image is waiting. The set of loci, walked in order, is your journey (or "route").