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Strictly speaking, that's true. When you have a RGB image, the channels simpy describe how much of each is in a particular pixel. If a pixel is at 0,0,0, it's black. When you view the channels, each one will also show a black pixel (0) at that location. If you have 0,0,255 the pixel is pure blue. The red and green channels will each have a black pixel, and the blue one will have a white pixel. If you have, 255,255,0 the red and green channels will each have a white pixel and the blue will have a black pixel. The composite image, though, will show a yellow pixel.
How is that useful? Well remember that you can copy a channel. Suppose you're trying to cut an object off its background. You could do it with quick mask, or the magic wand, or the pen tool, but if you've got a lot of detail in the edges, none of those things are likely to give you good results.
Instead, take a look at each of those color channels. It's fairly likely that at least one of them has pretty good contrast between background and foreground (assuming it was a decent photo to start with). If you copy that one and can find a way to somehow enhance that contrast until you have a black-and-white channel, you can use that channel as your mask.
Useful tools for achieving this are, of course, Levels and Brightness & Contrast under the Image > Adjustments menu.
You can also use the Burn and Dodge tools to enhance the edges. Set the burn tool's Range to "Shadows" and the dodge tool's Range to "Highlights" and brush them lightly across the contrasty edges. With a little finesse, you can even get a decent mask on frizzy hair or other difficult-to-mask objects.
If you can't find a single channel with enough contrast, check out Image > Calculations. This bizarre dialog allows you to combine two color channels into a new channel with different properties. Just experiment with different combinations and different blending modes until you see something that looks like a good starting point.
