In general, the number of channels refers to how many speakers the amp can run. One channel is one speaker. You can also wire up multiple speakers to one channel. There are various ways to do this.
If you have just one speaker, you can match the speaker to your amplifier
and you're all done!
Most of the time though, you will have more than one speaker especially when we're talking about subwoofers.
There are three ways to wire up your subs. The key is to choose a wiring
method which best matches the impedance of your sub combination to the optimum or recommended inpedance of your amplifier. If you amp wants to run 1000 watts at 4 ohms, chose a sub combination that gives you a 4 ohm load. It's that simple!