Plain English Meaning
A safe altitude shown on an approach chart that keeps you above all obstacles within a circle around a chosen point. It's there for emergencies, not for normal flying.
Full Definition
An emergency-use altitude depicted on instrument approach charts, providing at least 1,000 feet of obstacle clearance within a defined radius (typically 25 nautical miles) of a specified navigation facility or fix on the approach. MSA is intended for emergency use only and does not guarantee navigation signal reception or compliance with ATC clearances.