Equation of time is the difference between apparent solar time and mean solar time.
EOT = apparent solar time – mean solar time
Wikipedia defines EOT by the follwoing:
The equation of time describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of “reconcile a difference”. The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion along the celestial equator. Apparent solar time can be obtained by measurement of the current position (hour angle) of the Sun, as indicated (with limited accuracy) by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would have a mean of zero (https://en.wikipedia.org/wiki/Equation_of_time)
The following graph plots the values of EOT for the whole year: