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Tidal heating is heating from friction due to movements within a body due to tidal forces. The process is also called tidal working or tidal flexing. Tidal heating is one of the sources of heat in a body undergoing tidal force, for example, providing the heat to allow liquid water to exist in some solar system moons even though they are at sufficient distance that sunlight would provide insufficient heat.
If a body is not tidally locked with a body it is orbiting, then different parts of it face the host body (thus being closest to it) as time passes, and gravity is pulling significantly more on the portions of the bodies nearest each other, affecting the balance of forces that hold each body together, to the point that the bodies change shape by shifting mass. (Earth ocean tides are an example of such shifted mass, sea water displaced to the extent that the changes are easily discerned.) This shifting within each body is subject to friction and the heating that friction produces. On the other hand, if the same side of a body continually faces the other, then its internal mass is not shifting, but this is often not the case even if tidally locked: any eccentricity in the orbit results in at least a small change regarding which side faces the other body. Orbital resonances tend to increase or maintain such eccentricity, which, for example, has given some moons of Jupiter and Saturn sufficient warmth to harbor subsurface liquid water.