The card at the bottom is usually called the Shadow Card. It’s often used as a “check-in” card, kinda like a way to confirm the main message of the spread. For example, if the 10 of Cups shows up at the bottom, it’s probably pointing to something about family, close relationships, or the end of an emotional journey for the person asking.
Personally, I don’t think Shadow Cards really add much to a reading and can sometimes be distracting — especially for beginners. Like, I’ve seen people do relationship readings with cards like the 5 of Swords, Tower, and 8 of Cups, but then because the 2 of Cups is the Shadow Card, they’ll go, “Does this mean we’re getting back together?” No, it just confirms the reading is about relationships, not necessarily a happy ending.
That’s just my take, though. Some readers swear by looking at the bottom card and find it adds something to their readings. If it interests you, I’d say experiment with it and see what you think. But for me, I usually feel like it pulls away from the main message.
