Coordinated Universal Time or UTC (from the French, temps universel coordonné), is the standard by which the world regulates clocks and time. When expressing a time independent of any one time zone, UTC is the standard. It does not use Daylight Saving Time or "Summer Time".
Other time zones
Other time zones have their time expressed as an offset from UTC. For example, the Eastern Standard Time zone (EST - east coast of the US and Canada) is UTC-5, or 5 hours slower than UTC. During the summer, EST observes Daylight Saving Time, and becomes UTC-4. Israel time is UTC+2 (two hours ahead of UTC), or UTC+3 in the summer. India Standard Time (IST) is UTC+3.5 - 3 and a half hours ahead of UTC; there are several other time zones that are not an exact number of hours offset from UTC.
Greenwich Mean Time, or GMT, was used before UTC replaced it. They are usually the same time.
Relevance and when to use
Since Waze is a worldwide platform with editors from every time zone, it is sometimes confusing to coordinate map activities or check when an edit was made. Waze stores many time values in UTC format and converts for local usage. For example, turn restrictions are input and evaluated (for routing) in local time for the location of the segments, but may be stored in UTC.
For coordination activities, to avoid confusion, it is preferred to express times in UTC, or UTC and additional local time zones for the editors and map area. This is only needed if the editors are in different time zones, or the editors are in one time zone but the place being edited is in another time zone. This practice is especially needed for MapRaids, which can possibly include every time zone.
Tools
The Time and Date web site has tools to easily convert time zones or look up the time zone for a location. There is a UTC clock extension for Chrome.