Testicles form in the abdomen during fetal development. During the final months of development, the testicles gradually descend into the scrotum. If this descent isn't completed at birth, the testicle usually descends within a few months. If your son has a retractile testicle, the testicle originally descended as it should, but doesn't remain in place. Signs and symptoms of a retractile testicle include: 
The testicle may be moved by hand from the groin into the scrotum and won't immediately retreat to the groin.
The testicle might spontaneously appear in the scrotum and remain there for a time.
The testicle might spontaneously disappear again for a time.
 Retractile testicle is different from undescended testicle (cryptorchidism). An undescended testicle is one that never entered the scrotum.