Today we celebrate the immaculate conception of Mary. Catholics hold that Mary was conceived without original sin (hence, immaculate) For more information check out this post.
However, St. Augustine sums it up rather nicely: “"We must except the Holy Virgin Mary, concerning whom I wish to raise no question when it touches the subject of sins, out of honor to the Lord; for from Him we know what abundance of grace for overcoming sin in every particular was conferred upon her who had the merit to conceive and bear Him who undoubtedly had no sin." Augustine, Nature and Grace,4 2 (A.D.415).
Today is also a holy day of obligation in the US; so make sure to get yourself to Mass!