It is a ground-based safety net intended to warn the controller about increased risk of controlled flight into terrain accidents by generating, in a timely manner, an alert of aircraft proximity to terrain or obstacles.
[1] International Civil Aviation Organization Doc 4444 requires that radar systems should provide for the display of safety-related alerts including the presentation of minimum safe altitude warning.
An unusually rapid descent rate on a non-precision approach can trigger such an alert.
[3] It is worth mentioning that ICAO Doc 4444 does not provide a definition of the term MSAW.
Instead the term MSAW is ambiguously used in ATC community to identify such warnings as well as for data processing systems providing the alert function.