RSI DIVERGENCE is a difference between a fast and a slow RSI. Default values are 5 for the fast one and 14 for the slow one. You can use this indicator in 2 different ways:
normal RSI: check double or triple top/bottom on a chart meanwhile RSI is descending/ascending (check the example on chart)
signal line: when RSI Divergence cross zero line from bottom to top you get a buy signal (the line become green), vice versa when the RSI Divergence cross zero line in the opposite way you get a sell signal (the line become red)