Pilonidal means a 'nest of hairs'.
A sinus tract is a narrow tunnel (a small abnormal channel) in your body. A sinus tract typically goes between a focus of infection in deeper tissues to the skin surface. This means that the tract may discharge pus from time to time on to the skin.
This is a companion discussion topic for the original article at https://patient.info/skin-conditions/pilonidal-sinus-leaflet