Ah! An excellent question. Tectonic activity is one of the main processes behind the shaping of major landmasses. The surface of the planet is not a single uniform piece of land; were you to remove the extraneous details like water and human settlements, you would see lines delineating different 'plates'. Think of an insect with many different pieces of armor making up the whole of its outer shell.

[That's probably a relatable metaphor, right? One that Armin will be able to parse?]

In the case of my Earth and this planet as well, those plates are capable of movement and their particular behavior in relation to each other is described as plate tectonics. In some places they grind past each other horizontally -- this tends to cause earthquakes. In others they push apart from each other and here, as the Atlas describes, they form mountains by pushing together and upward. You can often mark a boundary between plates by either a mountain range or an area with an unusual amount of earthquakes.