UTC is a time standard by which the world regulates clocks and time. For most purposes it is synonymous with Greenwich Mean Time (GMT) but there are small differences.
Read all about it here.
In radio communications between an FBO and an aircraft, times that are given in UTC are often referred to as times in Zulu time. For example, 22:00 Zulu. The alternative in local time would be 22:00 local.
If you see and enter UTC or local time, is defined by a setting in your user login account. FBO One makes the translation based on the timezone that is defined in the country for the station that is set for your FBO location.