What does Luke 1:21 mean?

Zechariah is a priest, chosen to offer one of the two daily sacrifices of incense in the temple (Exodus 30:1–8; 40:26–27). He was alone in the Holy Place when he was visited by the angel Gabriel (Luke 1:8–12). Gabriel's message was uplifting. Zechariah and his wife, both aging (Luke 1:5–7), will be blessed with a child. That child will grow to be an important herald of the Messiah (Luke 1:13–17). In a moment of doubt, Zechariah questioned if the angel's message was true (Luke 1:18), so he was temporarily struck mute (Luke 1:20) and deaf (Luke 1:62).

Outside the Holy Place, in the courtyard of the temple, worshippers are waiting for Zechariah to come back. His task likely was not meant to take long. Whatever time it took for him to interact with Gabriel is enough to be noticed. Those waiting outside are curious as to what's keeping the priest. When he emerges, they seem to realize he's had a spiritual encounter (Luke 1:22). How much Zechariah can tell, or chooses to tell, is not explicitly stated.