Most likely it's just a number. However, given that Dutch is often fairly close to German and German for a bicycle is Fahrrad, it's probably basically "Bicycle route 28". That's just my guess, though.

In the Netherlands, this growing national network of "cycling highways" are being named and numbered just like the motorway system. In the case of the A28, the parallel car route, the "A" stands for "auto", which is Dutch for car. For the F28, the "F" stands for "fiets", which is Dutch for bicycle.