We have a bit of a twist to the previous declarations from God that no one will cross into the promised land before the generation that fled from Egypt had died. We actually get a specific age of 20 which we didn't have before. We see that anyone who was 20 and over when they left Egypt will not enter into the promised land, while those who were younger than 20 when they left Egypt would be able to enter the land.
But there were a couple of tribes who wanted to enter early into the promised land because their cattle wanted/needed more land and there was great land for their cattle in the area called the Transjordan, or across the Jordan. They made a deal with Moses, which was basically making a deal with God. Moses accused them of being opportunists thinking that they would wait for the other tribes to clear out the enemies and then they would just skate in and take the land that was promised to them.
They said no, here is a compromise. We promise to be a vanguard, those who go ahead of all the others and we will be the ones who battle the enemies as a sign that we are not afraid to enter the land and do battle even if our ancestors did say that we should not enter the land. We will wait to take over our land and settle in our land until all the other tribes of Israel have already settled in place. That seemed like a good idea, Moses was happy with it, the people were happy with it, and so, it seems, that God was happy with it as well. So now we begin to occupy the promised land after these many years in the wilderness.