In the early days, doctors thought pilonidal sinus was a dimple in the bum crack present since birth. For this reason, traditionally surgeons would perform a wide excision of the area. An open wound would then take months to heal. In some patients the condition still recurred!
This is an image from Google of what a typical pilonidal sinus looks like. The person is lying on their belly and the bum crack is what you see in the centre of the image. There are tiny holes (pits) in the crack that hair enters through. The hair then exits via the seconday opening (sinus) higher up and to the right in this example.
Ultimately, we now understand pilonidal sinus results due to a loose foreign hair being forced in through vulnerable skin. Successful surgical treatment depends on stopping the entrance of new hair.