Technically speaking, models are pre-compiled geometry parts which can be included in a map, if you didn't get that, they are objects, which can be used in your map, this includes stuff like chairs, tables, trees, you get it. These models are supplied with the game you make the maps for, not GtkRadiant itself, so they are different for each game which means you might not recognize any of the models used in the examples.

Models are usually made using modelling software such as 3ds Max or Blender, and for the id tech 3 engine, they usually come in the .md3 format. Using models, or even making your own can really simplify making your map, or at least visually spicing it up. The next chapters will teach you more about models.