The monitor is a small device that is attached to a human for observation purposes by the International Fleet. In Ender’s Game we see this device attached to Andrew Wiggin before he is recruited by the IF for enrolment in the Battle School.
The device is self-sustaining and connects to the host’s nervous system. Its primary purpose is to gather data of both the person being monitored and their environment. It is typically implanted for a period of three years.