I'll use an example of 09D00300.
The 09 is the year in which the rabbit was rung.
D is the size. Different breeds have different sizes.
00300 means the ring on your rabbits leg is the 300th 09D rung purchased.
The ring numbers are registered to their breeder, or a new owner if they have gone through the BRC and transfered the ring number into their name.
A vet should be able to remove the ring, or you can get a strong pair of wire cutters or tin snips. It's adviseable for rabbits that are pets only, as they tend to be overfed.