You are probably referring to a pilonidal cyst, which is a kind on skin infection near the tailbone, at the crease (or colloquially, "crack") of the buttocks. Typically, the infection causes a cyst or an abscess, and is quite painful. In some cases, the condition requires medical treatment, which may include minor surgery the cyst or abscess may need to be lanced or excised.
If your condition is not too severe, you can try treating it at home for a few days, but if you find no improvement you should be sure to visit a doctor. A hot compress should help - soak a soft, clean cloth in hot water, squeeze the cloth, and then apply it to the affected area for about two minutes or until the cloth cools. Do these as often as you can, if possible once every two hours. In addition, as with any other boil or cyst, keep the area clean by washing it gently. Due to the pain and discomfort this probably does not need to be mentioned, but avoid putting pressure on the cyst. Even once you get rid of the cyst, try to avoid putting pressure on the area for long periods of time, as this has been found to at least aggravate if not actually cause a cyst.
answered by G R