Urethra play is a specialisation of medical play and generally involves the insertion of a medical sound or catheter into the urethra. Due to the risky nature of this activity, it is often only performed by BDSM enthusiasts who are genuinely medically qualified, such as nurses and doctors.
There can be a strong dominance/submission element to urethra play as it actually involves literally penetrating the body in a very intimate way. In addition, when a catheter is used and it reaches the bladder, it takes away control of urinating from the submissive or bottom so they can no longer decide when or for how long they pee. It is instead up to their partner who controls this via a tap or clamp on the catheter itself.