Julian watches his grandfather repair watches is a sentence that describes an activity. It consists of a subject (Julian), a verb (watches), and an object (his grandfather repair watches). The subject is the person or thing that is performing the action, the verb is the action that is being performed, and the object is the person or thing that is receiving the action.
This sentence could be used in a variety of contexts, such as in a story about Julian and his grandfather, or in a description of a scene in a movie or TV show. It could also be used to illustrate a point about the importance of spending time with family, or about the value of learning a trade.