If there is no plug in the jack nothing is affected. If a plug is inserted into this jack, the signal comes out of the module instead of to the fader. This allows the insertion of outboard processors such as delays or noise gates between the mic preamp and the mix buss. These are usually before the fader. The circuitry that actually combines signals from the input modules is usually referred to as a buss.
Most mixers have several busses to provide a wide variety of features.
The labeling of each buss is according to the manufacturer's notion of which features sell mixers, but each buss works the same way electrically. If two busses are provided for a function, they are often labeled left and right. If there are more than two, they are numbered. Usually, each buss has a master level control somewhere on the mixer and an output on the patch bay. This mix allows the operator to listen to channels that are off as far as the main mix is concerned.