The urethra is the tube that carries urine out of the body from the bladder. A urethral stricture is an abnormal narrowing somewhere along this tube, caused by scar tissue that restricts the space urine has to pass through. It can develop from an injury, from a previous medical procedure that involved the urethra, from infection, or from certain non-infectious forms of urethritis (inflammation of the urethra).
Because the urethra is the only outflow path for urine, even a modest narrowing can noticeably affect how a person urinates — and a severe one can block the flow almost entirely.