开腔 is a separable verb (离合词) meaning 'to start speaking' or 'to open one's mouth to talk'. It is used in informal contexts, often when someone begins a speech or conversation. The verb 开 can separate from 腔 to insert aspect markers (了, 过), measure words (个, 次), or duration (半天). For example, 开了腔 means 'started talking', and 开个腔 means 'say something' (literally 'open a mouthful').