In the ABO and Rh blood group types (without getting into the 20 or so different types of rare blood antigens), people with type O Negative as their blood group are universal donors.
Simply put, if anyone needing blood walks into an emergency department, there is a more than 99% chance that they can be transfused safely with type O- blood without crossmatching. Time can be extremely limited in serious emergencies such as major trauma; formal crossmatching may not always be possible in time.
People with type O- blood who donate blood on a regular basis are lifesavers. Literally.
Type O is the commonest blood type amongst the A, B, O and AB blood groups. RH- people are rarer than RH+ people. O- people usually are the minority in a given population.