The Sadness card belongs to the Emotions family, with a yin polarity turned inward. It doesn't indicate failure, but a downward movement, a moment when something settles and seeks release. Sadness invites you to slow down rather than fight what's withdrawing, while a new clarity forms.
In Oradia's mirror-card system, Sadness answers to Joy: the two don't oppose each other, they answer each other, like two sides of the same movement.
This page gives the card's general meaning. In a real draw, the Sadness card is always read in relation to the other cards drawn and the intention set at the start — it's that combined reading, unique to each draw, that the online oracle develops.