When someone is hosting a playdate, they say which time zone it starts at. Basically, a time zone is an area that goes from the North Pole to the South Pole, and no matter where you are in that zone, it will be the same time. It is the same time in Chicago, Winnipeg, and Dallas. But since time zones can be annoyingly complicated sometimes and never neatly go north to south, I'll give you a crash course on how they work.
For example, let's say that I'm hosting a playdate starts at 8pm Pacific time, which includes British Columbia, the Yukon, Washington state, Oregon, and California. If someone from New York, which is in the Eastern time zone, wanted to join the playdate, it would start at 11pm over there. If someone from the United Kingdom wanted to join the playdate, they would have to wake up at 4am to play.
It has to do with the rotation of the planet. If it is daytime in Asia, it's night time in North America, since the two continents are on opposite sides of the globe. Try shining a flashlight at a globe in the dark, and you'll see why we need time zones.
Here's a time zone map. If an area is a different colour than the one beside it, it's a different time zone.