"Staff" is a word that means "employees" or "workers". People usually use the word "staff" when they're talking in general about the entire group of people who work somewhere:

The staff there is super friendly and helpful.

"Employees" is more often used when you're speaking from the perspective of the boss:

Have you ever had to fire an employee?

"Staff" is a collective noun, so you use it in singular form to talk about the entire group:

Afterwards, the entire staff went out to celebrate.

If you want to talk about just one or just a few people on the staff, use the phrase "staff members":

One of the staff members at my gym asked me out yesterday.