"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.