This means when one thing comes in, at least one must go out. There are obviously exceptions to this rule, but when out shopping somewhere like Target where we can easily get distracted and end up with a cart full of merchandise when all we really needed was kitty litter, instead of saying "oh this is so cute and so cheap, let's get it, it will be perfect!" we say hmmmm, is this something that a) we need b) we want c) we'll want in a month? If that doesn't knock the desire right out of our minds then this does: "Is this so great that I'm willing to let something else go to make room for it?" That's right. When one thing comes in the door another must leave. This is one way we keep our apartment from expanding at the waistline and keep clutter and 'things' from accumulating. Sometimes we'll keep it in the same family: one shirt comes in, one item of clothing comes out, new pillowcases in, old pillowcases out. But we get creative too. It can be fun to 'earn' a new item by working to create a space for it and to actually buy it on purpose instead of impulsively.