mean solar time on the 0° meridian passing through Greenwich, England, measured from midnight: formerly a standard time in Britain and a basis for calculating times throughout most of the world, it has been replaced by an atomic timescale
The links don't explain what "mean solar time" means, so the word "mean" is not yet explained.
"Solar" means based on the sun. So my guess is this:
If you measure a "day" by the sun, you measure from noon to the next noon, where each "noon" is the moment that the sun's shadow is longest.
Apparently that length changes by small amounts over the course of a year. The variation may be just a few seconds, but it exists.
So scientist take the "arithmetic mean" (a non-weighted average) of the 365 times, and call that the length of a day. Then each "mean solar day" is exactly that long, and the "Greenwich Mean Time" (GMT) is the time "at the astromy observatory in Greenwich England", counting in mean solar days.
(Unsurprisingly), the Wikipedia article for Greenwich Mean Time explains it:
Because of Earth's uneven speed in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the sun crosses the Greenwich meridian and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy calculated by the equation of time. Noon GMT is the annual average (i.e., "mean") moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".