This clip will explain some of it. Words couldn't do it justice. Don't forget to put your ear plugs in. 

Before they were allowed to operate casinos, the only legal gambling in Oklahoma was Bingo at gaming centers on Indian land. For poor and bored rural Oklahomans this was cheap entertainment that would sometimes pay off a few hundred dollars. 

In an (much) earlier post I included a link to info about the world's largest McDonald's on the Indian Nations turnpike. In Oklahoma it's as close as we're likely get to something like the Eiffel Tower. 

The "two-A" or double-A (football) Championship refers to a ranking system that groups high schools based on enrollment and geographical regions. AA is the second smallest ranking, indicating a rural or small town's high school. Like bingo, high school football is a major source of entertainment and pride for rural Okies. 

Cut-off britches = worn out blue jeans that have been repurposed as shorts by cutting the legs off. This guy's cousins have apparently cut them off very short, ala Daisy Mae, or more appropriately Daisy Duke (Dukes of Hazard TV show). It's hot and humid here in the summer, especially down in the boondocks. Skimpy clothing is de rigueur. Oh, by the way... Oklahoma has the highest rate of teenage pregnancy in the US. 

A pipe-rail gate is made from heavy duty, larger diameter (~4-6 inch) steel pipe and is HEAVY. It is usually hung from a steel post and pivots to block off a small road or driveway. In rural areas the driveway to a house can be quite long since the house on an acreage can be set back a distance from the main road. The singer is likening his manhood to a branch of a Bois D'arc tree (pronounced BO-dark in Oklahoma). This is extremely hard wood. Fence posts made of it last forever. The metaphor is obvious, but likening it to this wood takes it to the ultimate level. 

A Blue Cat is a variety of catfish, a lake-dwelling bottom feeder that can grow quite large (big as a bus according to local legend). When Uncle Slayton was a younger man he might not have relied on a trotline to supply him with fish. He probably would have done some hand fishing, or noodling. With over 600 miles of shorline Lake Eufaula, near his quarter-section, is prime spot for this type of activity. 

Hope this helps. Even in Oklahoma, rural areas like the one in this song are like foreign countries, the last remaining frontier outposts in America. They have a culture all their own. Outsiders are generally not welcome, but if you can gain their trust and respect you can find no better friends. Salt of the Earth.