Hair transplant hair follicle damage. (photos)

Abscess is an infection. It is a collection of pus that in built up under your skin.  You need to have this treated.

This seems to be an abscess and you will want to follow up with your Hair Transplant Physician as soon as possible. This can generally be caused when recipient sites were made too deep as well as the grafts themselves transplanted too deep which can cause  ingrown hairs that can lead to infected cysts and abscess.

I concur with the infection.  I recommend visiting with your hair transplant physician for further instructions.