In practice
Resistance forms where selling has repeatedly outweighed buying. Traders use it to plan: longs take profit into it, shorts look for entries when price shows rejection there, and breakout traders wait for a close above it.
Levels that are visible on higher timeframes, and that caused sharp reactions rather than slow drifts, carry the most weight.
What to watch for
Each visit uses up some of the sell orders, so a level touched many times is more likely to break than to hold. Buy stops collect just above obvious resistance, and price often spikes through to take them.