A high or low marks where relocation in one direction stopped during the period being observed. It records an outcome, not a guaranteed future barrier.
Full Explanation
A high is the price at which upward relocation stopped during the measured period. A low is where downward relocation stopped. Mechanically, each records a place where the interaction changed enough that price could no longer continue taking territory in the same direction at that moment.
That does not make a prior high permanent resistance or a prior low permanent support. The orders that produced the original turning point may no longer exist when price returns. New orders may have been added, old orders may have been consumed or withdrawn, and the size of arriving aggression may be completely different.
Highs and lows still matter because obvious structural points tend to attract attention, stops, limit orders, and other activity. They are useful places to watch. The mistake is treating the visible line itself as the cause of the reaction rather than watching what happens when current aggression meets current liquidity there.