... to recover of us by strong hand And terms compulsatory, those foresaid lands So by his father lost

Fortinbras sharks up his band of lawless resolutes with the apparent intention of reclaiming the land which should have returned to him upon King Hamlet's death. We learn in the beginning of the next scene, that this effort follows two months of petitions from Fortinbras for the return of what was probably understood by an Elizabethan audience to be legally his.

KING:Now follows that you know young Fortinbras,      Holding a weak supposal of our worth,      Or thinking by our late dear brother's death      Our state to be disjoint and out of frame,      Colleaguèd with his dreams of his advantage,      He hath not failed to pester us with message,      Importing the surrender of those lands      Lost by his father, with all bands of law,      To our most valiant brother. So much for him.  - (I.ii.17-25)

"So much for him.": While Claudius seems to dismiss Fortinbras' claim out of hand, he does dispatch his ambassadors, Cornelius and Voltemand to negotiate a settlement with Fortinbras' ailing uncle, old "Norway." The agreement is to buy off Fortinbras with a sum of 60,000 crowns annually (3,000 in the Second Quarto) at Norway's expense, which one assumes is in compensation to Fortinbras for the lost annual income from the contested land.

The more universal question embedded in Fortinbras' particular circumstance is "what should one do in a situation where he or she is unable to enforce a claim even though it appears to be valid?" For the Elizabethans, the answer appears to have been that a person should continue to assert their claim in the hope that the situation will eventually change. While "might makes right" might be the rule of the day, the Elizabethans had greater confidence in the rule of law

So if a man have right to land, and do not make his entry for terror of force, the law allows him a continual claim, which shall be as beneficial to him as an entry.  - Francis Bacon,"Maxims of the Law" quoted in Rappold, p.53

Ultimately, this confidence proves to be justified in the case of Fortinbras' claim.