Plain English Meaning
A message on the screen telling the pilot that a function is set up and waiting, and will switch on by itself at the right moment.
Full Definition
An indication shown on a GPS or flight management display confirming that a particular mode or function (most commonly the approach mode) has been selected and is now ready to activate automatically when the appropriate conditions are met. The system is staged but not yet engaged.
Context Anchor
Seen on a GPS navigator or flight display while loading, activating, or flying a GPS instrument approach.
Derivation
From the verb 'to arm,' meaning to prepare a device so it will act when triggered — the same sense as arming an alarm or a switch. 'Annunciation' comes from the Latin annuntiare, meaning 'to announce.' Together: the system is announcing that it is prepared and ready to act.