A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes.

Time zones tend to follow the boundaries of countries and their subdivisions because it is convenient for areas in close commercial or other communication to keep the same time.

This map shows you the different time zones and the letter designations.

This map from timeandate.com shows the time difference between UTC and other time zones.  https://www.timeanddate.com/time/map/

Before clocks were first invented, it was common practice to mark the time of day with apparent solar time (also called “true” solar time) – for example, the time on a sundial – which was typically different for every location and dependent on longitude.

Each time zone is 15 degrees of longitude wide.

The dividing lines for the time zones begin at Royal Observatory Greenwich, in England, also known as the prime meridian.

The time at Greenwich is call Greenwich Mean Time (GMT). Coordinated Universal Time (UTC) is the same as GMT. 

As you move west from Greenwich, every 15-degree section or time zone is an hour earlier than GMT, while each time zone to the east is an hour later.

With different time zones, no matter where you live on the planet, your noon is (roughly) in the middle of the day when the sun is highest.

Here is a table with the number of hours offset from UTC for each time zone

https://www.esrl.noaa.gov/gmd/grad/solcalc/timezone.html

Understanding time zones is important in meteorology as all observations and model forecasts are given in UTC time.