Plain English Meaning
The GPS uses the aircraft's altitude reading as if it were an extra satellite. This lets the GPS keep checking that its position fix is reliable even when fewer satellites are available.
Full Definition
Baro-aiding is a technique that uses barometric altitude information from the aircraft's altimeter to supplement GPS satellite data, allowing the GPS receiver to perform integrity monitoring (RAIM) with one fewer satellite than would otherwise be required. It substitutes a known altitude reference for one of the satellite signals normally needed to verify position accuracy.