I can't explain it any more precisely than this; so here you have it...
The Roman Catechism (Ad parochos, De bapt., 2, 2, 5) defines baptism thus: Baptism is the sacrament of regeneration by water in the word (per aquam in verbo). St. Thomas Aquinas (III:66:1) gives this definition: "Baptism is the external ablution of the body, performed with the prescribed form of words." Later theologians generally distinguish formally between the physical and the metaphysical defining of this sacrament. By the former they understand the formula expressing the action of ablution and the utterance of the invocation of the Trinity; by the latter, the definition: "Sacrament of regeneration" or that institution of Christ by which we are reborn to spiritual life. The term "regeneration" distinguishes baptism from every other sacrament, for although penance revivifies men spiritually, yet this is rather a resuscitation, a bringing back from the dead, than a rebirth. Penance does not make us Christians; on the contrary, it presupposes that we have already been born of water and the Holy Ghost to the life of grace, while baptism on the other hand was instituted to confer upon men the very beginnings of the spiritual Life, to transfer them from the state of enemies of God to the state of adoption, as sons of God. The definition of the Roman Catechism combines the physical and metaphysical definitions of baptism. "The sacrament of regeneration" is the metaphysical essence of the sacrament, while the physical essence is expressed by the second part of the definition, i.e. the washing with water (matter), accompanied by the invocation of the Holy Trinity (form). Baptism is, therefore, the sacrament by which we are born again of water and the Holy Ghost, that is, by which we receive in a new and spiritual life, the dignity of adoption as sons of God and heirs of God's kingdom.