Once a day NCEP runs 12 ensembles of the MRF forecast model. Each ensemble has a different perturbed initial state around a reference state (usually the Operational Run). These variable perturbed states are used to partially account for the fundamental problem of being unable to know the precise initial conditions.
MDL is producing experimental output based upon the forecasts from these 12 ensembles. This output describes the range of forecasts produced by the 12 runs for about 250 cities in North America. The forecasts provided are for high temperature, low temperature, and probability of precipitation for the upcoming 8 days. The Ensemble MOS page you came from provides three ways to visualize this otherwise complex (and sometimes confusing) set of data.
A description of each of these three display types is given NEXT.