Plot v. Content, as Johnstone put it in
Impro. The plot’s a sequence of things happening that connect back to earlier events and elements (without the latter, it’s not a plot, it’s just… stuff happening) that has a plain beginning, middle, and end, the flow of which most people can sense—he relates that children can usually tell when a story being made up on the fly is “over” and doesn’t demand to continue.
“Content” is theme and message. You don’t necessarily need narrative to have it, but it tends to exist in narrative also.
Johnstone advises, when writing or improvising, to worry about the plot, and that the content can take care of itself—it’ll just be there, like magic, when you look back on what you’ve created. Putting “content” first can weaken the narrative, and make one less creative, and anyway the content doesn’t need help to appear.
Of course, his advice is also explicitly for beginners, and he repeatedly mentions as an aside that of course one may take a more active hand in guiding these sorts of things as one gets good enough at the basic techniques that they become effortless.