First, it is a profession that there is only one baptism. As St. Paul writes in Ephesians 4:5, “there is one Lord, one faith, one baptism.” In other words, once you are validly baptized, you can’t be baptized again.
Second, it professes the forgiveness of sins administered in baptism. According to 1 Peter 3:21, baptism “saves us,” which implies it removes sin (both personal and original).
To learn more about this article of faith, read paragraphs 977-980 in the Catechism of the Catholic Church.