What is the SS58 account?
SS58 is a simple account format designed for Substrate based chains. There's no problem with using other account formats for a chain, but this serves as a robust default. It is heavily based on Bitcoin's Base-58-check format with a few alterations.
The basic idea is a base-58 encoded value that can identify a specific account on the Substrate chain. Different chains have different means of identifying accounts. SS58 is designed to be extensible for this reason.
The living specification for the SS58 account format can be found on theSubstrate wiki