The basic idea
Support is a price area where falling prices have tended to stop and bounce, as buyers step in. Resistance is a price area where rising prices have tended to stall, as sellers step in. They are among the most widely watched concepts in all of trading.
These levels form because of memory and psychology in the market. If a price bounced from a certain level before, traders remember it and place orders there again, which can make the level self-reinforcing — at least until it eventually breaks.
Zones, not exact lines
A common beginner error is treating support and resistance as precise prices. In reality they are zones, often several pips wide, because different participants see slightly different levels and because price frequently overshoots before reversing.
Drawing them as thin, exact lines leads to frustration when price pokes through by a little and then reverses, stopping you out for no real reason. Thinking in zones, and giving stops a little room beyond the obvious level, tends to work better in practice.
When levels break
Support and resistance do not hold forever. When a level finally breaks, an old resistance can become new support and vice versa, as the crowd's expectations flip. Trading these breaks is popular but tricky, because false breaks are common.
The honest position is that support and resistance describe tendencies, not certainties. They are useful for framing where to consider entries, stops, and targets — not for predicting the future. Combine them with risk control rather than treating them as guarantees, because any level can and eventually will give way.