They are actually a bit arbitrary but essentially because the earth rotates it is a particular time (say Noon) at roughly 24 different locations at different times of the day...
Everyone on the planet wants the sun to be at its highest point in the sky (crossing the meridian) at noon. If there were just one time zone, this would be impossible because the Earth rotates 15 degrees every hour. The idea behind multiple time zones is to divide the world into 24 15-degree slices and set the clocks accordingly in each zone. All of the people in a given zone set their clocks the same way, and each zone is one hour different from the next.
The sun is in a fixed position above the Earth...and, as we have 24K miles around the planet, it's easy / simple to divide it up into 24 time zones, so that each zone experiences the sun in the same relative position at the same hour.
Eg, "high noon" when the sun is at it's zenith, is the same (roughly) for everybody the world over...no matter what time zone you're in.
Does that help?
yes it does ty :)