UTC is an abbreviation for "Coordinated Universal Time". For most practical purposes, including shortwave listening, it is the time in Greenwich England. Because of this, it was once known as, and is still sometimes referred to as, "Greenwich Mean Time", or "GMT". In the eastern USA, UTC is four hours ahead of local time while daylight savings time is in effect, and five hours ahead when it is not.
Shortwave stations may be heard simultaneously over large parts of the globe; each area may have a different local time. Referring to these broadcasts using UTC allows everyone, broadcasters and listeners alike, to avoid the overhead and confusion of trying to translate time from one time zone to another. Because of this, broadcast schedules and listeners' signal reports, for pirates and non-pirates, usually use UTC. It is common for DXers to keep a clock at hand that is set to UTC, so that schedules can be observed and logs written down without having to translate local time to UTC. A clock can be set to UTC (or local time) to a high degree of accuracy by listening to the station WWV.