The prime meridian is an imaginary longitudinal line running through Greenwich England from the North to the South pole at 0 degrees longitude. This line divides the world into western and eastern hemispheres.
Also the prime meridian is the starting point for dividing the into time zones.
From the prime meridian or 0 degrees longitude each time zone is 15 degrees longitude wide corresponding to 1 hour of difference in solar time.
Travelling east of the prime meridian hours increase while moving from one time zone to another.
On the contrary as we travel west of the prime meridian hours decrease while moving from one time zone to the next.