Well, it's tricky to explain effectively. Essentially we use ensembles because we don't understand the uncertainty of our chaotic atmosphere. Taking into account this uncertainty, the model runs several times with slightly perturbed conditions in order to create the individual members in order to best account for all possibilities. That's how we get the ensemble mean. The ensemble control forecast is the ensemble member run from the unperturbed, coarse-resolution initial conditions. Essentially the control run is useless in conclusion