Computed: 22 × 21 / 2 = 231, the number of unordered pairs of the twenty-two letters; taken in both orders the pairs number 22 × 21 = 462.
Meaning
The 231 gates are Sefer Yetzirah's own image for the alphabet as a combinatorial engine: every pair of letters is a gate, and the book says the world was engraved with them. It is the first piece of explicit combinatorics in the tradition, and it is what lets the later Kabbalists claim as many as 231 substitution ciphers, one per gate.