Sure, you know how sometimes when you're running a session you want to leave some parts of the story to fate? With dice, you'd go like "The higher the roll, the more crowded is the nightclub tonight" and then you roll. Or you'd go "Odd, and the bookkeeper actually has a copy of the tome, even, he doesn't".
It's the same concept, but instead of getting a random number, you get a random concept, with a card. Then you can interpret the concept any way you want -it's meant to help with creativity and inspiration. For example, "... a mysterious black car parks right in front of you. The door opens, and who climbs down?" You draw a card. Say, "Ace of Crosses". You interpret that any way you wish, say -"It's Father Anselmo, the church's clean-up guy. By reputation he only gets sent when things get messy. You see him exchange a few words with the bouncer, then he's in through the back door". And so on. Unlike dice, you can arrange Tarot cards in various spreads, such as the Celtic Cross which sort of tells a story about the past, the future, the thing in play and the players. All sort of interesting things open up, but I wouldn't say the Tarot is a must-have. It's a nice-to-have, for a StoryHost.
There's 23 Major Arcana (a nod to Discordianism) and 56 Minor Arcana (8 suits of 7 cards each) plus a cover. I would say it's a nice-to-have, not really a must-have.