UT refers to a time scale called "Coordinated Universal Time" (UTC) which is the basis for the worldwide system of civil time, and is therefore the international time standard. This time scale is determined using highly precise atomic clocks around the world.
"Universal Time" is the current term for what was commonly referred to as Greenwich Mean Time (GMT). Zero (0) hours UTC is midnight in Greenwich England, which lies on the zero longitudinal meridian. (The standard of UT replaced GMT in 1926 because too many different definitions of GMT were in use.)
Universal time is based on a 24 hour clock, therefore, afternoon hours such as 4 pm UTC are expressed as 16:00 UTC.

The establishment of time zones accounts for the fact that for any given instance the Sun is rising in one place on the Earth, is standing high in the sky at noon for another place, and is setting for a third place. Considering these astronomical facts, it makes sense to use different civil time scales at different places on the Earth.
And since a day is 24 hours long, the world may be split into 15 degree wide longitudinal bands (360 degrees/24 hours) or time zones. Each band represents one hour. Therefore the local time of a time zone usually differs by a whole number of hours from Universal Time which coincides with the local time on the zero longitude. The difference "local time" minus "Universal Time" is positive (+) for time zones east of Greenwich and negative (-) for western time zones.

Daylight saving time is decreed for entirely political reasons and has no astronomical basis.