我是路人甲他是路人丙

Translates literally to 'I am路人A while he/she is路人C', '路人'in chinese refers usually to the general public or non-specific people in a setting, so it means in context I'm nobody special (extra A), and neither is he (extra C) (extras in filming context). People using this might express humility and commonality, suggesting that both them and another are regular folk with no particular importance or significance.