It's sacral dimple....In few cases there may b a underlying spine problem...can go for a lumbosacral imaging

Most of the sacral dimples are harmless and do not require further imaging. From the photo it appears as a simple harmless one. But do get it checked by your pediatrician in your next visit. In very rare cases, where the sacral dimple is large or has an associated skin tag or tuft of hair or skin discoloration , may have underlying spine problem. In such a case, child may have symptoms like tingling or numbness of legs or bladder/bowel incontinence.