The Julian day (not to be confused with Julian Date) is simply a count of the number of days since a specific start date. The start date was selected so that all of our historical period comes after that start date and so any date in history can be identified as having occurred on a positive Julian Day. Noon GMT on 1st January 4713BC was selected as the zero point. Julian days can have a decimal portion that indicates the fraction of the day that has passed since noon GMT. The reason for selecting noon is that this date format is commonly used by astronomers who would prefer to not have the day number change right in the middle of their work period.
This article written by Stephen Chapman, Felgall Pty Ltd.