Refers to a waistcoat-wearing, pocket-watch-wielding white
rabbit, hurrying along and muttering, "Oh dear! I shall be late!" in Lewis Carroll's Alice in Wonderland (1865), whom Alice follows down a rabbit hole into Wonderland.
Awesome rock band Jefferson Airplane's 1967 song White
Rabbit retold the story of Alice's adventures as though they were a psychedelic drug trip. But whether you take it literally or metaphorically, following the white
rabbit means following an unlikely clue, an innocuous, unbelievable (but also, frankly a bit ridiculous) sign, to find oneself in the midst of more or less extraordinary, marvelous, amazing circumstances that challenge one's fundamental beliefs, expand one's horizons &/or perception of realities, transform one's perspective, and change one's life.
The phrase has become commonplace in popular culture; e.g. in the 1999 film
The Matrix, the resistance fighter folks use the trope of following a white
rabbit to lead Neo/Mr Anderson/The One out of
the matrix. Oddly enough, though, the phrase hasn't filtered through as a metaphor in non-fantastical contexts to any appreciable degree. Outside of science fiction or fantasy, if a writer refers to the white
rabbit, s/he is almost certainly specifically alluding to previous uses, usually to Carroll. (We should change that!)
'There's something fishy going on here.'
'Well,' said Q, jerking her head toward the
door to the stairwell, through which the tattooed woman was disappearing. 'You'd
better follow the white
rabbit, then.'