MACD is basically the difference of 2 moving averages.
By default the averages in Zerodha Trader are (13-day EMA, 26-day EMA)
There is a signal line which is the average of MACD line,default value for it is 9.
Red line is the average of MACD and Blue line is the actual MACD. The most common way of using MACD is signal line crossovers, ie When MACD lines crosses above the signal line it is a bullish crossover and if it crosses below the signal line it is a bearish crossover.