Hermetic means "airtight", difficult or impossible for one of ordinary knowledge to comprehend, and "resistant to outside forces." In other words, like a hermit.
Does this sound familiar to you, fellow writers? When in the midst of crafting our respective universes, whether we're in a quiet space or a noisy high-traffic area, we are shutting ourselves off from the world in order to properly interact with new inner worlds. There will be minor instances of leakage (more inwards than outwards), but for the most part, these worlds are self contained until they're ready to be released. Like an egg hatching or a can opening.
This puts me in mind of Schrodinger's cat, a quantam mechanics gedankenexperiment (thought experiment) of the 1930s. Put a cat in a sealed box with a radioactive atom and a poison. Is the cat alive or dead? The thought is that the cat is both alive and dead until the box is opened and the cat is actually observed - the mechanism being tripped and the cat dead. This was not a serious experiment, by the way. It was never meant to be carried out, but was rather a discussion point Dr. Schrodinger had with his intellectual contemporaries on quantam physics, Einstein being one of them. From a creative standpoint, we as writers are a bit like Schrodinger's poor (albeit metaphorical) cat; the outside world has no idea what is going on inside our box (our hermetically-sealed creative world) until the box is forced open or... the cat itself comes out. I'm not saying "Stay in your box! Be a hermit! Reject all social interacts!" In fact, I see this as a natural state, when we're in the midst of our craft; a story or a novel will incubate as a mystery until it comes out into the world, and all bets are off. Stephen King uses the phrase "keeping the door shut" until a story is ready for an audience. Until then, embrace the hermitage. And afterwards, find your friends in the world and make them wonder what you're up to the rest of the day!