Support and resistance are price levels at which a market has repeatedly stopped and reversed: support is a level below price where buying has historically absorbed selling and halted declines, while resistance is a level above price where selling has historically capped advances. They are the most basic reference points in technical analysis.
The levels come from prior swing highs and lows, round numbers, session highs and lows, and prior consolidation edges. The working idea is that traders remember these prices and place orders around them, so they tend to react there again — until they do not. A decisive break of resistance often turns that level into new support (and vice versa), the “polarity” or role-reversal principle.
Support and resistance are zones, not exact lines, and they are partly self-fulfilling and partly hindsight — every level looks obvious after it holds. Turning them into a strategy requires fixed rules for how a level is defined, what counts as a touch, and what counts as a break, so the idea can be measured rather than eyeballed.