A square links two planets 90° apart, almost always in signs of the same modality (both cardinal, both fixed, or both mutable) — which means both planets want to act in their own, incompatible way at the same moment, with neither naturally willing to yield.
It's the aspect most associated with real friction, but also with real growth: a square rarely resolves on its own, which is exactly why it tends to force the kind of change a more comfortable aspect never would.