In Numbers 36 some leaders from the tribe of Manasseh come to Moses with a concern. The five daughters of Zelophehad, as yet unmarried, would be inheriting land in Canaan (Num 27). But what if one or more of them married someone from another tribe? Then that land presumably would go to the husband's tribe and would be lost to the tribe of Manasseh (verse 3).
The Manassite clan leaders add in verse 4, "And when the jubilee of the people of Israel comes, then their inheritance will be added to the inheritance of the tribe into which they marry, and their inheritance will be taken from the inheritance of the tribe of our fathers."
The verse has led to lots of questions, since it is not clear why the jubilee is referenced here. According to the jubilee rules laid out in Leviticus 25, the jubilee would restore land that an owner had lost for financial reasons. These rules did not cover who that owner should be. The jubilee seems irrelevant to the issue at hand.
Commentator Michael Morales explains that in the traditional interpretation of the passage, the point being made is that not even the Jubilee, which is intended to set things right with regard to land ownership, could fix the problem they are worried about.
Moses, under God's direction, rules that the daughters of Zelophehad, and any other women who will be receiving plots of farmland, should marry within their own clans. The daughters of Zelophehad are happy to comply, again prioritizing the interests of their tribe and clan above their own interests.
Traditionally it is believed that the ruling of Numbers 36:5-9 was a special ruling for those in this generation, according to the reasoning that once the land was distributed under Joshua, the tribe that controlled each parcel of land would be fixed from then on.