成亲 means 'to get married' in a formal or traditional register — you often hear it in historical fiction, royal dramas, or rural settings. It's a separable verb (离合词): you can insert aspect markers or measure words between 成 and 亲, like 成了亲 ('got married'), 成过一次亲 ('got married once'). Modern everyday Chinese prefers 结婚, so 成亲 carries a classical or narrative flavor.