A support level is a price zone where buyers have repeatedly stepped in, preventing further decline. A resistance level is the opposite — a zone where sellers have repeatedly capped rallies. Once broken, resistance commonly becomes support and vice versa.
Strong S/R zones are typically identified by multiple historical touches, high traded volume at the level, and round numbers (1.1000 on EUR/USD for example). They are the most widely used tool in technical analysis because they describe actual order clusters rather than derived math.