Relative Strength Index

The relative strength index (RSI) has at least three major uses. The indicator moves between zero and 100, plotting recent price gains versus recent price losses. The RSI levels therefore help in gauging momentum and trend strength.
The most basic use of an RSI is as an overbought and oversold indicator. When RSI moves above 70, the asset is considered overbought and could decline. When the RSI is below 30, the asset is oversold and could rally. However, making this assumption is dangerous; therefore, some traders wait for the indicator to rise above 70 and then drop below before selling, or drop below 30 and then rise back above before buying.
Divergence is another use of the RSI. When the indicator is moving in a different direction than the price, it shows that the current price trend is weakening and could soon reverse.
A third use for the RSI is support and resistance levels. During uptrends, a stock will often hold above the 30 level and frequently reach 70 or above. When a stock is in a downtrend, the RSI will typically hold below 70 and frequently reach 30 or below.