What it means
Draw it as a zone, not a thin line. Each test uses up buy orders, and obvious levels collect stops just beyond them.
In practice
Support forms where buying has repeatedly outweighed selling. Traders use it to plan: shorts take profit into it, longs look for entries when price shows rejection there, and stops on long trades go below it.
The best levels are obvious on a higher timeframe and produced a strong bounce the last time they were tested.
What to watch for
Support is a zone and it weakens with every test. Sell stops sit just below obvious levels, so a quick dip under support that recovers is common. Buying only because price is at a level, with no sign of buyers, is catching a falling knife.