What does "stopped down" mean in photography?

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 The aperture blades are fully withdrawn into the lens housing and do not restrict any light from passing through the lens.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: