1. what is an EOS account?
Instead of having only public and private key like other blockchain, EOS user must have an account name. An EOS account is a readable name that is stored in the EOS blockchain and connected to your keys. An EOS account is required for performing actions on the EOS platform, such as sending/receiving tokens
2. What's the rule for it?
It must be exactly 12 characters long and consist of lower case characters and digits up until 5.