Like stochastics, RSI measures momentum of price movements on a scale of 0 to 100.
Always confirm RSI signals with other indicators. RSI can remain at lofty or sunken levels for a long time, without prices reversing course. All that means is that a market is quite strong or weak - and likely to stay so for a while.
Adjust your RSI to the right timeframe for you. A short-term RSI will be very sensitive and give out many signals, not all of them sustainable; a longer-term RSI will be less choppy. Try to match your RSI timeframe to your own trading style: short-term for day traders, longer-term for position traders.
Divergences between prices and RSI may suggest a trend reversal. Of course, make sure you confirm your signals before acting.