Promises and vows
"A vow is a deliberate and free promise made to God concerning a possible and better good which must be fulfilled by reason of the virtue of religion," A vow is an act of devotion in which the Christian dedicates himself to God or promises him some good work. By fulfilling his vows he renders to God what has been promised and consecrated to Him. The Acts of the Apostles shows us St. Paul concerned to fulfill the vows he had made.
Source: Catechism of the Catholic Church (Vatican.va)