签到 is a separable verb (离合词) meaning 'to sign in' or 'to register one's arrival'. The verb 签 and the object 到 can be separated by aspect markers (了, 过), measure words (个), or time expressions, e.g. 签个到 ('sign in once') or 签了到 ('have signed in'). It's commonly used in workplaces, schools, or events to record attendance.