GMT stands for Greenwich Mean Time. This is a special time that is used as the standard time all over the world. It is named after the city of Greenwich in England because that is where the official time is set.
When people talk about time zones, they are talking about the difference between their local time and GMT. For example, if it is 2pm in New York, it is 7pm in London (because London is five hours ahead of New York, and is on GMT).
GMT is important because it is used as a reference time for lots of things, like international flights, TV broadcasts, and scientific experiments. So, everyone uses it to make sure they are all talking about the same time.
Overall, GMT is a really important way to keep time organized and consistent all over the world so people can communicate and coordinate with each other.