UTC stands for Universal co-ordinated time. UTC is an official world-wide atomic clock time standard. National laboratories around the world have atomic clocks synchronised to this atomic time standard. Leap seconds are introduced at pre-defined intervals to compensate for variations in the earths rotation.
Leap seconds allow UTC time to closely match UT or Universal Time which is synchronised to the earths angular rotation, rather than a uniform passage of time.
UTC atomic clock time is also sometimes referred to as Zulu time.