From a medical perspective, a graft is a piece of living tissue that is transplanted surgically. In hair restoration context, a hair graft is the surgical transplantation of a follicle or follicular unit into the scalp.
A follicle is a single hair from a group or follicular unit. And, a follicular unit is a group that can have multiple hairs, usually between 1 to 4 follicles.
So, when referring to an FUE Hair transplant, a graft is usually meant to explain the surgical process of transplanting a follicular unit or group of hair and not just one follicle or one hair.
For example, 100 grafts can equal the transplantation of 100 follicular units but its highly unlikely that would only contain 100 hairs, because 1 follicular unit usually grows between 1 to 4 hairs so in reality it would be anywhere from 100 to 400 hairs.