UTC stands for Coordinated Universal Time. It is the reference clock the whole world measures against. Every time zone on earth is described as a number of hours ahead of or behind UTC: New York in winter is UTC−5, Berlin is UTC+1, India is UTC+5:30.
The important thing about UTC is that it never changes. It has no summer time and no winter time. That stability is exactly why aviation, shipping, computer systems and international teams use it.
When should you use UTC?
Use UTC when you are writing to people in several countries at once and you want zero ambiguity. "The release goes live at 14:00 UTC" means the same instant everywhere, and nobody has to guess whether you meant standard or daylight time.
For a conversation with one other person, naming a city is usually kinder: "3pm your time in Manila" is easier to act on than an offset.
UTC and time on this site
Every clock and conversion here is calculated from the same global time zone database that operating systems use, then expressed relative to UTC. That is why daylight saving shifts appear automatically on the correct dates for each country.