cjg - it means that of those being tracked in the subject group (Bavi + Doce and then continued to PDx)...
More than half of those that belong in that group are still alive.
mOS means Median Overall Survival and signifies the point at which the middle patient (say 6th of 11) have passed. At that point - the duration of survival of the 6th is the mid point (5 died in less time on the trial, remaining 5 will die with longer duration) of the experiment.
It does not tell you the range (not in the info they gave) or number/quantity of patients.
It also does not share distribution. There can be 5 patients that passed after 3 months of treatment, 6th that lasted 8 months, and 5 others that are around after 20 months... So, mOS does not give distribution - just middle point.
Best,
MH