Landmarks are elements that a person won’t necessarily interact with, but provides an orientation cue and makes locations more memorable. Landmarks are usually visible from many locations, and change in some predictable way relative to a person’s movement through space.
In the real world, famous structures like the Eiffel Tower are visible from many areas, and a person can tell their orientation in the city relative to it and other landmarks in the city. On a smaller scale, a large building in a neighborhood will serve a similar purpose. Grocery stores often use huge signs on the walls for the different departments that line the edges of the store. These help if people are looking for the department that they’re naming, but they also keep them oriented as they wind through the aisles looking for items.
In the digital world, things like a logo in the top left corner, or a global search box are landmarks. Landmarks that change in predictable ways, like consistent breadcrumbs, and active states in persistent navigation are even more useful, because they make the user’s position relative to other areas of the site clear.