Support and resistance levels are used by technical analysts to identify price points on a chart where there is a probability for a pause or reversal of the current trend.
When a downtrend is expected to pause due to a concentration of demand, support occurs.
Resistance occurs when an uptrend is expected to pause for a short period of time due to a concentration of supply.

Support indicates a low level reached by a stock price over time, whereas resistance signifies a high level reached by a stock price over time. Support occurs when the price of a stock falls to a level that encourages traders to buy. Reactionary buying causes a stock's price to stop falling and begin rising. In contrast, resistance appears when a stock's price rises to a level that causes traders to sell. This selling causes the price of a stock to stop rising and begin falling.