You will often hear some photographers say “shoot wide open”. What does it mean? Generally, the words “wide open” refer to the size of the lens aperture – it literally means to “shoot at maximum aperture”. For example, if you are shooting with a lens like the Nikon 50mm f/1.4, its maximum aperture is f/1.4. If I said something like “The center of the frame looks sharp wide open”, you can rephrase this sentence to “The center of the frame looks sharp at the maximum aperture of f/1.4”.
The largest circle in the above illustration represents the “wide open” aperture of f/1.4.