Very roughly it is the luminance from the lamps divided by the background luminance from the room's walls and ceiling.
(For an explanation of what luminance means, this book...
...will help you)
Have a look at this detailed explanation of the formula for UGR:
Log is log10 by the way.
Lb, the background luminance or cd/m2rd, L is the luminance of the luminaire.
Looking at L and Lb, glare increases with stronger lamps and lower background lighting, whereas it decreases with weaker lamps and more background illumination.
p is the Guth index, which increases with distance from direct line of sight. So as the lamp moves further from the line of site, p increases and so the glare, as measured by the UGR, decreases.
Some useful to know limits:
- UGR < 10 : Glare is so insignifigant it can be ignored.
- UGR > 30 : Lots and lots of glare!
Note that there are many UGR ratings for the same luminaire but in different rooms. The table above gives UGRs for rooms with different wall, ceiling and worksurface reflectancies, as well as different room sizes (described as dimensions in MH (mounting height) units.)
The luminaire whose UGR is shown above is quite a low glare device, most of the room dimensions sizes give a value of around 10, which is considered a good value.
Notice also that since the luminaire is rotosymmetrical whether it is viewed endwise or crosswise does not change the UGR much. It's a different story with an assymetrical luminaire: