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Crustal plates are sections of the Earth's crust that shift, earthquakes often being the settling of plates that have been stretched or otherwise deformed from shifting in relation to neighboring plates. The mechanism (and the well-accepted theory that it holds true on Earth) is called plate tectonics, and the occurrence of the shifting is called tectonic activity.
The question of whether some other world (moon, solar system planet, or extra-solar planet) experiences this activity is of interest, both simply to understand the planet/moon, and because on Earth, plate tectonics appears to be part of the Earth's system that maintains a life-supporting climate, i.e., its presence may be a factor in whether an exoplanet could support life anything like Earth's.
Of interest regarding bodies beyond the snow line (e.g., some exoplanets, as well as moons of Jupiter and Saturn) is that the body's crustal plates can be frozen water (or some other frozen volatile material), yet shift in a manner analogous to Earth's crustal plates. The same could theoretically be true of Earth's polar regions.