Universal time (UTC) is the same as Greenwich Mean Time (GMT), give or take a second. It’s essentially the time in Greenwich, England except it ignores Daylight Savings Time.
The abbreviation UTC is an odd compromise. The French wanted to use the abbreviation TUC (temps universel coordonné) and the English wanted to use CUT (coordinated universal time). The compromise was UTC, which doesn’t actually abbreviate anything.
Sometimes a ‘Z’ is appended to a time to indicate it is expressed in UTC. The NATO phonetic alphabet code for ‘Z’ is ZULU, and so UTC is sometimes called “Zulu Time.”
It’s useful to know how your time zone relates to UTC. (You can look it up here.) For example, I live in the US Central time zone. Central Standard Time (CST) is UTC-6, i.e. we’re 6 hours behind Greenwich. Knowing your time relative to UTC makes international communication easier. It also helps you read computer time stamps since these almost always use UTC.
One of the advantages of UTC is that it avoids Daylight Savings Time. DST is surprisingly complicated when you look at it in detail. Some countries observe DST and some do not. Countries that do observe DST may begin and end DST on different dates, and those dates can change from year to year. And inside countries that observe DST some regions are exceptions. For example, the United States generally observes DST, but Arizona does not. Actually, it’s even more complicated: The Navajo Nation inside Arizona does observe DST.