John Gruber, after reporting that the name “Marzipan” isn’t this project’s real code name, and before saying that he doesn’t expect to see this at WWDC 2018:
I don’t have extensive details, but basically it sounds like a declarative control API. The general idea is that rather than writing classic procedural code to, say, make a button, then configure the button, then position the button inside a view, you instead declare the button and its attributes using some other form. HTML is probably the most easily understood example. In HTML you don’t procedurally create elements like paragraphs, images, and tables — you declare them with tags and attributes in markup. There’s an industry-wide trend toward declaration, perhaps best exemplified by React, that could be influencing Apple in this direction.