Plain English Meaning
The navigation system watches itself. It keeps measuring how accurately it knows where the aircraft is, and if that accuracy drops below what the procedure needs, it tells the pilots so they can take action.
Full Definition
A required capability of Required Navigation Performance (RNP) systems in which the aircraft's navigation equipment continuously checks its own position accuracy against the performance standard required for the procedure being flown, and warns the flight crew if that accuracy can no longer be assured.
Context Anchor
Seen in Required Navigation Performance discussions, especially for routes and instrument procedures that depend on the aircraft meeting a stated navigation accuracy.
Derivation
Two ideas combined: '' (Latin monere, to warn) means keeping a continuous watch, and 'alerting' means raising a when something is wrong. The pairs them because watching alone is useless without telling the , and alerting alone is useless without something watching first.