There’s also an in-game “Rules Guide” available from the top-right menu
The event system is pretty complex. The event deck starts out seeded with some random beneficial events. When you play an event card, you discard it face down and then reveal the top card from the event deck. When the event deck runs out of cards, all those discards are shuffled to make a new event deck.
So basically you are helping to form a future event deck when you play an event card. You know what will happen, but you can’t be sure when.
The event screen tries to summarize what you know. It shows the cards that have already been revealed on the right. It shows the cards that you placed in the current event deck in the middle, and reminds you how many unknown cards (placed by other players) are still there. And to the left it shows the same information for the future event deck (i.e. current discard pile). There are arrows to give you an idea of the order that cards will be coming up. That’s important because you will often want to prepare for known future events by building military, economy, etc.