The DRAW_CANDLES drawing style plots the candles using the values of 4 indicator buffers with Open, High, Low and Close prices.
The indicator plots the candles on two symbols in the separate window. The N variable is defined as input parameter, it can be changed using the "Properties" window. The color and symbols changed randomly each N ticks. To see how does it work, attach two indicators to the chart.
See also: The Drawing Styles in MQL5.