My husband and I *love* to walk ...for hours and hours, and if we decide to go ahead with this potential trip - I'm going to need good shoes. These days, I have some reasonably good shoes for the most part - but they are 2-3 hour walking shoes, not necessarily good for 6+ hours of tromping around.
In cell biology, selective permeability is the property of a living cell membrane that allows the cell to control which molecules can pass through the membrane, moving into or out of the cell.
The cell membrane is selectively permeable, meaning it only lets certain things in and out of the cell. If you think about our building analogy, this can be a very important thing. You wouldn't want just anyone running in and out of the White House, for example.