Two of the tribes, Reuben and Gad, see the land of Gilead, and decide it would be a good place for them, because their herders and it looks like fine grazing land, rather than going on to Canaan. Moses asks why their brethren should go to war and not them (because they don't want to?) and accuses them of causing the others to lose heart, just like their fathers did. He reminds them how god cursed them for that, and how no one who was over 20 at the time is going to see the holy land because they've been wandering in the desert for 40 years. He says god will do it again if they leave.
They propose to build houses and sheep pens where they are, fight the battle, and come back. Moses agrees to those terms. The sons of Reuben and Gad set about building their cities and fields.
This chapter consists of a list of all the places the Israelites have camped over the past 40 years. It's about as interesting as looking at other people's vacation photos: 'Oh look! Here's Elim! Remember, dear, how there were 12 fountains there and 70 palm trees! That was a good camp! Oh, no! Rephidim! That one was awful! No water! No dear, the worst was Edom. Remember how Aaron died there? Oh, yes, dear, you're absolutely right. That was terrible.'
After 50 verses, Moses comes back with some new instructions: conquer the Canaanites, destroy their property, drive them out. Failure to do so will result in prickly eyes and thorns in their sides, in addition to whatever he was planning for the Canaanites.