The ensembles are the whole set of runs that the models calculate. Basically they alter the starting conditions VERY slightly and see what that does to the runs. THis creates a big set of possible runs. If the operational (the one we see first - the main one) is supported by a lot of the other runs, then that's a sign it might be on the right track; if it's an outlier (i.e., has little support) then they might suggest caution is needed.. either that or it's a trendsetter!
I don't use NW's model viewing page - I use wetterzentrale, meteociel, and weatheronline (expert(, as well as the TWO viewed sometimes. There are links on all of these to the ensembles for London, Manchester, Aberdeen.. and meterociel have clickable maps for ensembles for anywhere.. but I keep forgetting the link for that!