These are parameters from the curve known as “Secp256k1”. s is a number that goes along with the random point. It's calculated from z which is the hash (number) of the message we want to sign. k is the random number we introduce each time for our signature. r is k times G. I had never seen x before interiawp started their forum career, so it may be unofficial. They mean the private key, but I know it's symbolized by the letter d.

The signature is [r, s].