What is the importance of Baptism?

The Catechism of the Catholic Church (paragraphs 1257) states,

The Lord himself affirms that Baptism is necessary for salvation. He also commands his disciples to proclaim the Gospel to all nations and to baptize them. Baptism is necessary for salvation for those to whom the Gospel has been proclaimed and who have had the possibility of asking for this sacrament. The Church does not know of any means other than Baptism that assures entry into eternal beatitude; this is why she takes care not to neglect the mission she has received from the Lord to see that all who can be baptized are "reborn of water and the Spirit." God has bound salvation to the sacrament of Baptism, but he himself is not bound by his sacraments.

Who can be baptized?

Baptism is given to those who having not yet been baptized, and who, having expressed faith in the Trinity and in Christ, desire to be baptized into Christ for the remission of their sins (Cf. Acts 2:38).

The use of water and the Trinitarian formula with the proper intention accomplishes this even outside the Catholic Church. Thus, the Catholic Church does not rebaptize validly baptized Christians being received into the Church.