That's the Focal length To Diameter Of the Dish Ratio.

The value tells you have deep or shallow your dish is and how the feed will illuminate the dish.

There are pros and cons to shallow dishes, they achieve theoretically higher gain than a deep dish, however they have a tendency to pick up more noise than a deep dish do.

Deep dishes have typically approx F/D Ratio of 0.25 to 0.30, they pick up less noise, however it is extremely difficult to make efficient feeds at this ratio, Seavey Engineering have feeds at this ratio and they work very good to my experience.

I used to install the american Winegard KU-band mesh dishes in the late 80's which came in 1.8, 2,4, 3.1 meter with F/D of 0.287 which are pretty deep.

My experience with those dishes was that they performed much better than equivalent dishes with higher F/D ratios like 0.40 or similar, they were the best dishes i ever had my hands on for consumer use if properly installed with Seavey feed.

The feed means everything so it can illuminate the dish properly.