镇痛 is a separable verb (离合词) meaning 'to relieve pain'. It combines 镇 (to calm/relieve) and 痛 (pain). You can insert other elements between the two syllables, like 镇了痛 ('relieved the pain') or 镇一镇痛 ('relieve the pain a bit'). It's commonly used in medical contexts — you'll see it in 镇痛药 ('painkiller') and 镇痛效果 ('pain relief effect').