Understaffed describes a place of work that does not have enough people working in it — either fewer than it needs to do the job properly, or fewer than it would normally have. It is said about the organisation, not about the people: a hospital, a shop, a shift, a department, a school is understaffed. You would not call a nurse understaffed. It is nearly always used with a note of complaint or explanation behind it — it is the reason the queue is long, the reason the ward is unsafe, the reason nobody answered the phone. Because of that, it often appears in the same breath as its partner word 'overworked' ('understaffed and overworked') and with 'chronically', 'badly', 'seriously', 'dangerously'. The word usually points at management or funding rather than at bad luck: saying a place is understaffed quietly says somebody did not hire enough people. Grammatically it behaves as an ordinary adjective — 'the clinic is understaffed', 'an understaffed night shift'. The related noun phrase people reach for is 'staff shortage', which is more common in headlines than the word 'understaffing'.