The DRAW_FILLING drawing style plots the filled area between the values of two indicator buffers. In fact, it plots two lines and fills the area between them with the specified color. The indicator plots a channel based on two moving averages with different periods in the separate window.
The change of the channel color shows the change of the trend. The colors of the channel changed after N ticks. The N variable is defined as input parameter, it can be changed using the "Properties" window.
See also: The Drawing Styles in MQL5.