It refers to the Oklahoma Land Rush of 1889. Roughly 50,000 people lined up on the borders of modern Oklahoma to claim their “homestead” of up to 160 acres (free land, pre the Homestead Act of 1862. At high noon on April 22, 1889. They lined up on the border until a cannon was fired (Boom) to start the rush.
Only 2,000,000 acres were available and if all 50,000 people had successfully claimed their full 160 acres, then that would indicate a “need” for 8,000,000 acres, or four times what was available.
Arithmetic was not unknown in those days, so some stealthy times snuck into the Unassigned Region early and staked their claim on the best (mostly, watered) lands on time, but having arrived early, were called Sooners.
Two types of Oklahoma settlers. Boomers were on land slated to be given away (or sold at incredibly cheap prices) before it was even given away, ie, squatters. Sooners were the folks who jumped the gun early to get choice plots when the giveaway was announced but were not on the land previously.