Price zones where supply exceeds demand (resistance) or demand exceeds supply (support).
Example
Ethereum bounces three times off around €3,000 — that zone becomes support, because buyers repeatedly step in there. When price finally breaks below, the role flips: old support becomes resistance (a support-resistance flip). Traders place stops just beyond these zones and plan entries at them. The more often a level is tested and the higher the volume there, the more significant it is.