The Door card, Revelations family, yin polarity, evokes an opening taking shape within a wall. It signals a possible passage where you only saw an obstacle. This card invites you to notice that opening, even a partial one.
In Oradia's mirror-card system, Door answers to Key: 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 Door 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.