Was reading this answer (emphasis mine):
It also tells us that the lens was stopped down, as if it were wide open there would be no corners to cause diffraction, regardless of the number of aperture blades.
What does "stopped down" mean?
This picture shows a lens at different apertures. I used a manual focus lens (a Soviet-built 50mm f/1.9) because having aperture controls on the lens itself made it easier to take pictures of the lens at different apertures.
In the top third, the lens is wide open, or at its maximum (widest) aperture (lowest f-stop value), in this case f/1.9. The aperture blades are fully withdrawn into the lens housing and do not restrict any light from passing through the lens.
In the bottom right, the lens has been stopped down about one stop to f/2.8. It is no longer at its widest aperture: you can see the aperture blades have physically extended into the lens opening slightly, to restrict some of the light coming through the lens (if I understand correctly, one stop cuts the light coming through the lens by half; this lens has been stopped down by just over one stop (from f/1.9 to f/2.8; normally one stop would be from f/2.0 to f/2.8) so the aperture blades should actually be covering just a little more than half of the area of the lens).
In the bottom left, the lens has been stopped all the way down to f/16, it's smallest aperture (and highest f-stop value). This lens has six aperture blades; in this portion of the picture you can see three of them meeting.
(I apologize for the different exposure of the bottom-left photo. I was shooting in Av, not Manual; perhaps the different reflection of light from the fully-stopped-down lens convinced my camera to up its exposure by 1/3 stop. I'll leave this as a reminder to readers to shoot in manual whenever you're combining multiple photos)
Here is a picture of the lens at f/16 to see what it looks fully stopped down:
It means the diameter of the aperture stop (a physical object in the centre of the lens which literally stops light to increase depth of field and decrease exposure) was reduced (by the photographer either via the camera body or directly on the lens) some amount from its widest setting.
"Stopped Down" simply means "with a small aperture". The aperture is expressed as a fraction, for example, f/8 or f/2 -- f/8 (the aperture is physically an eighth as wide as the focal length) is smaller than f/2 (the aperture is half as wide as the focal length)
It does not mean "with a small aperture"; it means that the lens is at a smaller aperture than it is capable of. f/1.8 for example is not a small aperture; a 50 mm f/1.8 would be wide open at f/1.8 but a 50mm f/1.2 would be "stopped down" at f/1.8. Conversely, f/5.6 is really a rather small aperture, but an 18-200 f/3.5-5.6 would be wide open at 200 mm f/5.6; a 70-200 f/2.8 would be stopped down by two full stops at the same setting.
(Ah, crap. Was meant as a comment to another answer. But it works as an answer in its own right too so never mind...)
If you have a typical lens which starts with Aperture of F3.5, then any increase in F stop, say to F4.5, would mean you are stopping down the lens.
Confusion continues to reign about Fstops and the terminology 'stopping down' It is a factor when listening or veiwing Youtube videos and the contributor says "I stopped down the lens" without giving an explanation for those that don't know what that means.. It literally means 'increasing the number on the Fstop dial/indicator upwards by number e.g.: fstop 2.8 to Stop 3.4...on upwards. Of course increasing/"stopping down" also decreases the the size of the diameter of the aperture which results in reducing the amount of light reaching the sensor, that in turn means your getting less light because your shutter speed will be faster. So you need to adjust your shutter speed to a slower period to allow and compensate for more light needed as you 'stop down' then and even further. ISO can be adjusted too..but can result in higher 'noise' or grainer image quality.