What is a lifter?

[SoulKeeper]

I have a 2000 GT 4.6L. I have posted numerous times on here explaining my problem I am having with my car. I took it to a Mechanic down the street from me. He told me that the tapping noise I have, sounds like a lifter. Either a lifter or a Roller Rocker. But most likely he said is a Lifter. So what exactually is a Lifter? He also told me that he would be willing to fix the problem for $400. Is that a good price or no? He is going to put a stethascpoe on it sometime early this week to pin point where the noise is coming from then go from there. I will replace all of the lifters or rockers when the work gets done. And hopefully I can help him in assisting fixing the problem. Any info on what a Lifter is would really help me understand this. Thank you in advance.