“Remember the word that Moses.” “Remember” is an infinitive absolute in Hebrew and is idiomatic for “keep remembering,” or, “keep in mind.” It is not a one-time remembering, but a keeping in mind (cp. Ex. 13:3). The tribes of Reuben, Gad, and half of Manasseh had livestock, and when Israel conquered the area east of Israel that was east of the Jordan river, those tribes came to Moses and asked not to cross the Jordan River, but to be able to settle where they were, on the east side of the Jordan, because that was good grazing land. Moses allowed them to do that on the condition that the men of those tribes would cross the Jordan with the other Israelites and fight to conquer the Promised Land, and they agreed to that (Num. 32).
“this land.” Joshua is referring to “this land,” the land they were all standing on, on the east side of the Jordan River. God said the tribes of Reuben, Gad, and half of Manasseh could stay on the land east of the Jordan, called the land “beyond the river.” But the men had to help the rest of the Israelites conquer the Promised Land west of the Jordan River.