Resistance is just opposite of support. As the word itself says to resist or stop something. Resistance level on a chart depicts a level where the rising price of a stock/index will be halted. The reason for any stock to move downward from it resistance level is due to heavy sell-off at rising price.
When stock rises people who have bought it lower will sell, and the demand of buying gets reduced due to high price. The resistance level is always above the current market price.