“Yeah, well they've certainly added a lot of staff recently.”
During your lunch break at work, you're chatting with a coworker about one of your company's competitors. Your coworker is worried about the competitor and mentions that they are getting bigger. You say this because you're not as worried about them, but you do agree that the competitor has added more staff.
Yeah, well they've certainly added a lot of staff recently.
This phrase is a way of agreeing while also disagreeing. When you say "Yeah, well...", it means that you can't disagree with the actual statement that someone just made, but you don't agree with their overall idea. For example:
A: I beat you!
B: Yeah, well, I'll get you next time.
"Certainly" is another word that you use to agree and disagree at the same time. The word "certainly" means "definitely". But it's often used to show a small feeling of disagreement or dissatisfaction. For example, if someone is acting a lot happier than usual, you can say:
You're certainly in a good mood today!
This sentence contains a very small suggestion that there's something strange about the person's good mood. Another example is:
I certainly hope he's able to finish in time.
Part of the meaning of this sentence is "I hope he's able to finish in time, although I don't think he will."
"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.
To "add staff" means to hire more employees for a business.