A mono AMP (one channel) means that the AMP is set up to control one thing (sub). A 2 channel AMP means you can hook up 2 things (subs) into it. A 4 channel means you can hook 4 things (subs) into it. Each channel has output holes for the + and - wires. Now here is where it can get a bit confusing. Lets say you have a setup for 2 subs. You have the following options, you can get a 2 channel AMP and put one to one and the other to the other. You can get a 4 channel AMP and bridge, so now you got one sub in 2 channels and the other in the other two channels. Or you can get a mono AMP and run the subs in Parallel. There are many different options. MAKE SURE YOU HAVE THE RIGHT THINGS MATCHED UP AND YOUR IMDEPENDANCE IS CORRECT (especally for 2 subs to mono AMP configuration). If i missed anything or got something incorrect somebody will help too. That is just a rought outline.