It means that the satellite's orbital period is equal to the Earth's rotation period ... roughly 23 hours 56 minutes 4 seconds ... so that the satellite always stays over the same longitude.
IF that satellite's orbit has also been carefully placed over the equator, THEN the satellite is in a 'geostationary' orbit. It appears motionless in the sky, and can be used to sell 900 channels of TV to people with little dishes that they never need to move.