The numbers are represented by right angles and parallel lines. Here are two diagrams that I put together using ink, scissors, a scanner, a flash drive, and Skitch.
Now that you know how each number is represented, let's explain how the brand works.
The brand has three main parts: The breed brand, the year of birth, and the registration number. There are quite a few breeds that have a brand for this system, but I only know of 19 (18 shown here) at the moment.
(Sorry about the blurry handwriting!) Anyway, that's that. And actually I missed one of the freeze breed brands - the one for BLM Mustangs. I think it looks something similar to a "U"? You can look it up if you like. :)