Resistance is the opposite of support. Prices move up because there is more demand than supply. As the prices move higher, there will come a point when selling will overwhelm the desire to buy. This happens for a variety of reasons. It could be that traders have determined that prices are too high or have met their target. It could be the reluctance of buyers to initiate new positions at such rich valuations. It could be for any other number of reasons. But a technician will clearly see on a price chart a level at which supply begins to overwhelm demand. This is resistance. Like support, it can be a level or a zone.