A shadow card is usually the card at the bottom of the deck after you’ve finished pulling cards for your question or spread.
It tends to represent the underlying energy of the situation — something influencing everything else, an unseen factor, or an extra layer to keep in mind. Sometimes it can feel like encouragement or a general message that ties the whole reading together.
What it represents can depend a lot on the context of the reading, but I usually think of it as a clarifying card that helps explain or deepen the rest of the spread.