An aperture is an opening, usually a small one. “If you can thread the silk through that aperture and pull it out the other side, we can knot it and create a loop.”
The most common place you’ll find aperture is when you’re talking about cameras or photography. In that case, an aperture refers specifically to the hole or opening in the lens that lets light through, which you can adjust (with the f-stop) to let in more or less light, resulting in a shallow or deep range of focus. A large aperture means only one tiny point of the photo will be in sharp focus, a small aperture means that much of the background as well as the foreground will be in focus.