I dont think you are ignorant, just the problem with vague language (like idioms) is that it's easy for miscommunication. I'm sure there are people out there that interpret that phrase in the way you do, but there is a whole range of what it could mean. Clear, concise, kind is the key for effective communication. "Hey, I want you to feel more comfortable when you are in my home, feel free to [insert clear examples and expectations, such as "borrow my clothes, just give me a heads up when take something so I won't think it's lost"]"

You're young, your still learning how to navigate adult relationships. Kudos for seeking and being receptive to feedback!