复婚 means 'to remarry one's former spouse' — it's a separable verb (离合词) where 复 and 婚 can split: you can insert a time duration or measure word between them, like 复过一次婚 ('remarried once'). The prefix 复- means 'again' and 婚 is 'marriage', so it specifically refers to remarrying the same person you divorced, not just any remarriage.