补牙 means 'to fill a tooth' (dental filling). It is a separable verb (离合词) — the verb 补 and the object 牙 can split when you add aspect (了, 过), measure words (一次, 三颗), or other modifiers between them. For example: 补了一次牙 ('had a filling once'), or 补过牙 ('have had a filling'). The specific tooth being filled is usually introduced with a preposition (在) or as a topic, not placed directly after 补牙.