A time picker is a control for choosing a time of day, such as 9:30 in the morning. It may show a clock-like menu or let someone type the time. It is similar to setting an alarm: the person chooses when something should happen.
For example, an appointment form might ask someone to pick an available meeting time. Showing whether the time is morning or afternoon can prevent mix-ups when a 12-hour clock is used.
Use a time picker when the exact time matters. Tell people which time zone applies if the time is shared across places, since 9:30 can mean different moments in different parts of the world.