String action refers to the height of the strings above the fretboard on a guitar or other stringed instruments. It affects how easy or difficult it is to press the strings down and play notes. Lower string action can make playing easier, especially for beginners, while higher action may produce a louder sound but can be harder to play. Adjusting string action involves raising or lowering the bridge or nut of the instrument. Proper string action is essential for achieving optimal playability and sound quality, allowing musicians to perform comfortably and effectively.