Okay, so, some of the guys from the tribes of Reuben and Gad have a lot of cows. And they've noticed that some of these newly conquered lands that Israel is acquiring are just perfect for cattle-herding. Coincidence? They think not.
So, the Reubenites and the Gadites approach Moses, Eleazar, and some leaders in the community to ask if they can just hang here and set up shop with their families. Who needs this whole Promised Land thing, right?
Moses is a little annoyed. So, these guys expect everyone else to head out to war over the Promised Land while they sit there milking cows? Don't they remember what happened last time people didn't want to bother heading to the Promised Land?
Let us refresh your memory: God made the whole group wander in the wilderness for forty years until nearly everyone died off.
Moses calls them a "brood of sinners" to cap off his whole rant.
But the Reubenites and Gadites ain't got time for that. They promise that they will help Israel fight to get to the Promised Land, as long as their wives, children, and livestock can stay behind in these non-promised lands. They also won't take any of this new land, they'll just head back home to the other side of the Jordan River and keep hanging with their cows and families.
Oh, well, if they'll fight, then that's a different story.
Moses makes the men swear that they'll follow through with their promise or else God will get them. Somehow. God's pretty good at that, so we're sure he'll follow through.
So, Moses gives them the land that was formerly the kingdom of the Amorites and they build it up (along with half the guys from the tribe of Manasseh.) Naturally, they give all the towns different (and way cooler) names.