The Reorder texture allows you to reorder channels of an input color. By default, Channel Order is set to rgba which means it leaves the input color unmodified.
To modify the reorder of the input, simply type in Channel Order the new channel order. For example rrrr will output the content of the input red channel for each rgba color output components. Channel Order also supports 0 or 1 in place of the channel name. For example if you want to output a white color you would type 1111, and 0000 for a black one.
For example, let's say we have an input color (0.25, 0.8, 1.0, 0.17) and we specify as Channel Order a0r1. The output color will then be (0.17, 0.0, 0.25, 1.0)