[2021] I'm really enjoying this game! But what's burnt exactly?

Offerings are "burnt" when used. Basically, they're consumable. You use them, they're gone. The ones that send you to a specific location do nothing in the beta, as only the McMillan estate is available. But in the full version, they'll let you pick the map when you burn them (assuming someone else doesn't burn one for a different map too).