Why do we do that? Catholic Traditions Explained

What is holy water and how do I explain the reason that it is placed at the entrances of churches to my non-Catholic friends?
Holy water is a sacramental; a reminder of the life of the church expressed and celebrated through the sacraments. By blessing ourselves with holy water as we enter the church, we are reminded of our own baptism and our call to conversion. Much as we are baptized and then journey through life, we bless ourselves and move through the church, symbolizing again the guiding presence of the Lord, which comes to us through baptism. We especially use holy water during Easter when the priest sprinkles the people. It serves to remind us that we are indeed on a journey of faith that makes us one with Christ. So blessing ourselves not only reminds us that we are baptized into a community of faith, but also helps us renew our own baptismal promises to reject sin and to believe in God. It is the heart of Christian belief to turn from sin and toward God. Holy water and baptism make it a visible sign for all of us to follow.