A point-of-interest (POI) is a place on the map geo-located at a specific latitude/longitude. Most POIs are also associated with particular locations on a road, so you can select POIs as routing destinations -- or at least drive close enough to a POI to walk to it from a road or parking lot.
There are many categories of POIs, including (but not limited to):
A broad category such as restaurants typically includes subcategories such as fast food chains, seafood, steak houses, vegan food, ethnic food of assorted types, neighborhoods, and much more.
For routing purposes, you can typically search for POIs by category, subcategory, name, street address, phone number, web site, and so forth. You can even search for POIs of a given type within a specific distance of your route -- such as fast food restaurants within 5 km of a longer route.
Depending on the type of POI, additional details can be provided on the map. For example, a public parking lot could include some or all of the following details in addition to its location: