It breaks some of the hardened salt (or sugar, or whatever crystalized substance) keeping the lid from turning. It doesn't work if the lid is stuck because of something viscous instead of hard; for those, a prolonged application of medium force works better than a very strong force of short duration.

There are two reasons. The soft tap. Loosening particles (sugar, salt etc) that form a rough surface.

Second one is the slap. Inertia makes the contents move the lid relative to the jar breaking the seal. No more vacuum, less friction, easy access.

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Jars are often sealed with a partial vacuum inside. Tapping can release this vacuum (creating microscopic channels for air to equalise the pressure) thus reducing the friction from the lid being pushed against the body of the jar