The satellite signal containing information on its position in its orbit that may be prone to error.
The orbit of a satellite around our planet is easy to describe mathematically if both bodies are considered point masses, but in real life they are not. For the same reasons that the Geoid is not a simply shaped surface, the gravitation pull of the Earth that a satellite experiences in orbit is not simple either. Moreover, satellite orbits are also disturbed by solar and lunar gravitation, making flight paths slightly erratic and difficult to forecast exactly.
A control station can obviously compare results of positioning computations such as those discussed above with its accurately known position, flagging any unacceptable errors and potentially labelling a satellite as temporarily “unhealthy” until those errors have been corrected and brought back within the agreed tolerance limits. This may be done by uploading a correction to the clock or the satellite’s orbit settings.