Refers to involuntary urination during sleep, typically used for children. The term is straightforward and not particularly euphemistic.
My child's bed-wetting just won't stop.
As a measure against bed-wetting, I limit fluids before bedtime.
Compound of 寝 (ne, 'sleep') and 小便 (shōben, 'urine/urination'). The reading ねしょうべん is a straightforward combination.