The Relative Strength Index is an indicator used to determine if a stock or asset is overbought or oversold. The indicator consists of a line that moves between 0 and 100. RSI line is plotted usually by using 14 (standard but adjustable) previous candles worth of average price changes.
When the RSI line crosses into the area below 30 the stock or asset is deemed to be oversold, indicating a possible reversal. The same goes for when the RSI line crosses into the area above 70, the asset or stock is overbought and a possible reversal is going to happen.
Image source: Disnat
For more information (also the source) visit: Investopedia and Disnat