The baptistery is the first area of the church proper - it houses the baptismal font - where people are brought into the Family of God (the Church) through the celebration of the Sacrament of Baptism. Sacraments are outward signs instituted by Christ and given to the Church. Through the celebration of Sacraments divine life is bestowed on us. There are seven Sacraments, the first of which is Baptism. Baptism is the way out of the kingdom of death into life, the gateway to the Church, and the beginning of a lasting communion with God. The word "baptism" comes from Latin and means "washing."
Once baptized, a Catholic enters the church building to join their fellow parishioners to worship and thank God for the gift of Jesus' death and resurrection in the celebration of the Mass. In due time Catholics receive the Sacrament of the Eucharist - or Holy Communion - the Body and Blood, Soul and Divinity of the glorified and risen Jesus, which comes to us from the altar after being consecrated by the priest. Note that the baptismal font and the altar are on the same axis, reminding us that Baptism and Eucharist are central to our Sacramental life.