If you still don't understand what an overflow is, I'll try to explain.
Basically they are boxes in the back of the tank, that are slightly shorter than the top of the tank. So if you fill up the tank to the top of the overflows, nothing happens, but if you fill it up more, the water flows over the top of the boxes, and to the bottom of the overflow. At the bottom of the overflow there are usually holes, which carry water out of the tank and into a sump system. This would be a very easy problem to fix - just put pvc caps on top of the holes in the bottom of the overflow.
The reason that CO2 and overflows don't mix is because, essentially, water will splash down from the top of tank to the bottom of the overflow, causing lots of disruption in the water. When this happens, CO2 can escape, which is something you don't want to happen in a planted tank. However, if you just put a cap over the hole at the bottom of the overflow, you can easily fill up the tank, as well as the overflows, so that no water will be leaving the tank, and there will be little to no surface disruption.
Hope this helps.