The Relative Strength Index's (RSI) main role is to identify the relationship between the price and the real offer/demand. Spotting areas where coins are at the peak of their strength, or bottom of their weakness is where this indicator becomes incredibly useful.

The market provides an overbought signal when the RSI line crosses the 70 and an oversold signal when the RSI line crosses 30.

Overbought = Sell signal
Oversold = Buy signal

The actual brains behind RSI is the simple formula 100 - 100 / (1+RS)
RS= Average gain of price rises / average loss of price falls (during the time frame specified)