过门 is a noun referring to a musical interlude or transition, especially in traditional Chinese opera or songs. It can also mean the opening bars of a piece. As a verb phrase (过门, guò mén), it means 'to pass through a door' or 'to get married (of a woman moving into the husband's family)', but the noun sense is the focus here. Note: the standard pinyin for the noun is guòmén, though the source lists guō.