John Gill Commentary Joshua 1:15

"until Jehovah have given your brethren rest, as [he hath given] you, and they also have possessed the land which Jehovah your God giveth them: then ye shall return unto the land of your possession, and possess it, which Moses the servant of Jehovah gave you beyond the Jordan toward the sunrising." — Joshua 1:15 (ASV)

Until the Lord has given your brethren rest, as [he has
given] you
Rest from their travels, as they had:

and they also have possessed the land which the Lord your God gives
them :
are settled in the land of Canaan, as they were on that side Jordan:

then you shall return unto the land of your possession, and enjoy it ;
the countries of Sihon and Og, they were put into the possession of:

which Moses the Lord's servant gave you on this side Jordan, toward
the sunrising ;
the land, given to them lay to the east of Jordan.