Support is a price area where demand has previously been strong enough to halt a decline, and resistance is an area where supply has been strong enough to halt an advance. These levels are identified from prior reaction points, round numbers, or zones where volume has concentrated.
The idea behind them is that participants remember past price levels and place orders around them, which can cause price to react when those levels are revisited. A level that is broken can switch roles, with former resistance sometimes acting as support and vice versa.
Support and resistance are observational reference points rather than fixed barriers. Levels can hold, break, or be tested repeatedly, and they describe areas of interest rather than certainties about future movement.