UTC is Coordinated Universal Time, this is the proper term. GMT may still be used. Zulu maybe used with military, aeronautical & maritime time & doesn't consider Daylight Savings time.

To identify/coordinate locations & time around the world - we use the Equator (Earth's horizontal center) and the Prime Meridian which is a vertical line (N to S) which goes through Greenwich, England (within London), which is 0 degrees longitude. This vertical line and 180 degrees longitude, on the other side of the earth, divides the Earth into the Eastern (Europe > Australia) & Western (So. America, U.S. > Hawaii) Hemispheres.

UTC is the same as London's Time (excluding DST).  If Greenwich/London time is 8:00, then UTC is 8:00 in Memphis, Phoenix, Germany or Japan. Events worldwide can be coordinated.

To help understand this, do a search for 'Standard Time Zones Map' to see the 'hours' difference between the Prime Meridian and another location. Also look at the Circle Converter where you can look at the current central time and see what the UTC time is, or work it the other way to forecast a local time to UTC,

Wikipedia's time zone map is at  World Time Zones Map - Time zone - Wikipedia .   Right click & 'save image' = 1920 pixel wide, if magnified with two '+' clicks should make the map save able at 4000 pixels wide.

In the Western Hemisphere (Atlantic Ocean & U.S. side) our local time is less than UTC because the Sun rose in England before it does in the U,S.

The opposite in the Eastern Hemisphere (Europe to Japan side) India local time is greater then UTC because the Sun rose in India before it will in England.  

There are six 'one hour' time zones from the Prime Meridian in England to Memphis TN.

To find out what the current UTC in Memphis is:  If it's 12:00 you would add 6 hrs and UTC is 18:00 (6:00PM). If it's DST then add only 5 hrs.

If you wanted to participated in a Memphis event that starts at 15:00 UTC, then you need to subtract 6 hrs (5 if DST) and be ready at 9:00AM Memphis Standard Time.

Versus, if you want to set the time for a 9:00AM Memphis event in UTC then you need to add 6 hrs (5 if DST) & tell everyone be ready at 15:00 UTC or 1500 UTC.