To put it very simply, a King like Charles I believed absolutely that he was born to be king. Being king "by accident of birth" meant, to him, that he had been chosen by god. Afterall, by the accident of birth anybody could be king. That he was king meant that he had been chosen to be king, therefore by divine right. This also meant that, as he was god's chosen, he wasn't answerable to any person, as he was only subject to god. Barry Coward's "The Stuart Age" is a good introduction to this.