mostly to restate applefani, but to elaborate:
Geometry refers to the screens ability to accurately display shapes on the screen. For example look at this pattern.

If you were to display this pattern on your monitor you would check the geometry by looking to see if each of the circles were as close to perfectly circular as possible, and the squares should each be exactly the same size with no bowing or shearing in any direction. Each line should be perfectly straight horizontally and vertically.
On an LCD display the geometry is usually 99.99% perfect.
Missing OSD merely means there is no digital on-screen adjustment like many tv's and monitors have these days. Everything is handled by buttons on the front of the monitor, and some controls also on the inside of the monitor.
On a good CRT the geometry will not be perfect, but it can be made very close. I would say my Sony P1110 CRT monitor has something like 98% accurate geometry after my fine tuning.
TV's and other CRT technologies are usually worse than that, especially in the corners. To partially compensate for this manufacturers set their screens to have overscan, which will push the boundaries of the screen past the visible portion. This puts the better geometry closer to the visible edges of the screen, but sacrifices intensity and resolution.
HOWEVER
Don't worry about it. You've probably been looking at displays with worse geometry than the PVM's all your life and never even realized it. The geometry patterns really bring out problems that you would probably never have realized were even there without the pattern. During gameplay you will still think things look the right shape and size, and the image on a PVM will look fantastic.
Additionally, all the PVM's I've ever seen had excellent geometry compared to the field of CRT's available. It's only when compared to an LCD monitor or another range of pro CRT's like the XM series that they have "poor" geometry.
On a personal note, I have owned an XM-29 plus, AM-3501R, XC-3717C, Diamondscan 20m, PVM-1371Q, PVM-1390, PVM-2530, and several arcade monitors and I prefer the PVM-2530 out of all them for it's great color and clarity while still having not too high of screen pitch.