`Staffing` is about supplying an organization with the people it needs to run. As a verb ("to staff"), it means to be those workers or to provide them: "volunteers staff the phone lines at night", "the clinic is staffed by two nurses". Notice how often it appears as "is staffed by" — the passive is the natural home of this word.

As a noun, `staffing` covers the whole question of how many people you have and whether it is enough: staffing levels, staffing costs, a staffing shortage, a staffing agency (a company whose business is finding workers for other companies). It is a management word, an office word, a word from meetings and news reports about hospitals and schools. When a manager says "we have a staffing problem", they mean there are not enough people, or not the right people, or people are leaving faster than they can be replaced — the word politely wraps up all of that.

There is also "staff up", used when an organization is deliberately hiring more people: "they're staffing up before the holidays".