The Normans were originally Norse Vikings who raided along the northern European coast and up rivers including the Seine. At the Treaty of Saint Clair-sur-Epte they were given land in Neustria to stop them from continually raiding France, particularly Paris, and in return for their conversion to Christianity and Rollo pledging allegiance to the king of France, and the marriage of one of the daughters of the French king to Rollo.
Its a little bit more complicated then that. The granting of Normandy to the Normans was the last in a long line of grants of lands to viking raiders by Kings of the Franks to act as a safeguard against other viking raiders in general, not the specific raider. Examples are, Harald `Klak', for instance, the Danish king baptized in 826, Rorik, the holder of a Frisian benefice in the 850s, or Godfrid, who was granted Rorik's territory in the 880s. How effective this was is open to interpretation, as there may have been too many raiders and not enough converts.
There is a good article called
From poachers to gamekeepers: Scandinavians and the Carolingians that explores this.
Abstract:
This article represents the first comprehensive study of the commendation
and conversion of Viking leaders by Carolingian rulers, from
the first recorded instance under Charlemagne to the agreement with
Rollo in the early tenth century. The survey underlines how
widespread the practice was, and permits an assessment of its effectiveness
as a defensive strategy against Scandinavian incursions. The
outcome varied: some Scandinavians found themselves defending
Frankish territory against Viking attack, others acted as intermediaries
between Franks and Scandinavians, still others were granted
Frankish benefices but never trusted, and ultimately killed. Nonetheless,
the article demonstrates that in the majority of cases the practice
of commendation and conversion worked to the Carolingians' advantage,
neutralizing potential enemies or even turning them into useful
allies.