Here's an analogy that may help when thinking about probability tables: Many types of standard codes like Morse code are like Huffman coding, where you give different variable length codes for each letter.. In Morse E and T are both a single dot and a dash respectively but X and Z get four.

You can view this Huffman Table or Morse codebook as a probability distribution on the likely letters to follow. A symbol with a small number of bits == high probability. A symbol needing large number of bits == low probability.

The only innovation with Arithmetic coding and ANS is that they allow for "fractional bits:" you don't need a whole dot for a very likely next-letter. But the idea is the same: you are taking your understanding of the probability distribution table and using it to come up with codes for the following letters.

One caveat is that instead of agreeing to a fixed code upfront, you agree to an algorithm to dynamically estimate the future probability tables as you read through the file.