Support is a level where buying has previously been sufficient to stop a decline; resistance is where selling has stopped an advance. Drawn on a chart they are horizontal lines through prior turning points, and the everyday claim is that price tends to react at them again.
There is a mechanism, and it is worth stating because it explains when the idea works and when it does not. A level where a lot of volume traded is a level where a lot of positions were opened. Holders who bought there and watched price fall are, on a return to their entry, offered the chance to exit at break-even, and many take it — supply appears. Traders who missed a bounce place orders to catch the next one. Stop orders cluster just beyond obvious levels, so a break through one triggers a cascade. None of this is mystical; it is order flow left behind by past participation.
That explains why the strongest levels are those where the most trading occurred, not those with the neatest touches. A level tested many times on heavy volume represents a large accumulated stack of positions. A level defined by two wick tips on a quiet afternoon represents almost nothing. It also explains the observed tendency for a broken level to change roles: the supply that was sitting there has been absorbed, and the participants trapped above it are now positioned differently.
Round numbers behave as levels for the same reason. Traders place orders at them because they are memorable, which concentrates orders there, which makes reactions at them more likely. The number has no significance; the clustering does.
The discipline that separates useful lines from decoration is refusing to redraw after the fact. A level identified before a test is a hypothesis. A level identified afterwards, adjusted to fit where price turned, is a description with no predictive content — and it is what most chart illustrations show. Drawing them as zones rather than as single prices is also more honest: turning points cluster within a range, and a one-tick line implies a precision the market does not have.
In crypto two extra cautions apply. Levels differ between venues, because prices differ between venues, and a level drawn on one exchange's chart may not exist on another. And a market that trades continuously through every weekend and holiday produces more failed tests than one with a session structure, so a break that reverses within hours is common enough that a single close beyond a level proves little.