Chevy Mailbu repair question?

My son is buying a 2001 Chevy Mailbu today. 102K miles. Seller says the lifters need repair but you could drive it for "3 years even" before they're fixed. My question, noy being car savvy, is what exactly are the lifters? I mean I know they make a tapping noise but thats about the extent of my knowledge. Is this an expensive repair job? P.S. Payin $1800