I want to believe

[Rafe]

The question asked was in essence what does a disbelief saving throw mean?

What does it mean when you pass and what does it mean when you fail?

Passing should be obvious, even in the OP's game (hence the "should"). As for failure... that conversation could go on for another few [dozen] pages. Each group of players and DM needs to determine that amongst themselves. Personally, I've always hated illusions and their implications in play.

In the situation the OP described, there shouldn't have been a save needed, so I, personally, feel the debate is moot in this particular context. Obviously, there may need to be larger discussion on illusion in general.