The most common thing you will want to do with channels is to insert an Alpha Channel. Because the Alpha value specifies how transparent the image is you can then use this to perform blends and mattes when drawing onto other images.
Generally you start with two images. One contains the picture and the other contains the Alpha Channel. You can then insert the Alpha Channel into the picture.
To make use of the Alpha channel you must draw in transparent mode otherwise it will be ignored. You can combine two pictures with alpha channels using this mode but in general you normally want to just combine one picture with another using the Alpha channel as a matte.
Sometimes you will hear alpha channels referred to as masks or mattes. Typically a mask or a matte is a reduced quality alpha channel containing only 0 or 255 opacity values.