basically the code is everything back end non graphical that makes literally everything on computers and machinery work, it is the instructions on how to display things and what to do when certain actions are taken that the computer or machine reads

open source code can be audited personally by the user, so it is very consumer friendly and a vouch of confidence by the creator to just leave it open source and allow anybody to see; but most big companies like to keep it proprietary and not release the source code and harrumph something about security (which could be fixed by making it open source so people could tell them the security risks for FREE)