We understand that we are saved by grace alone, through faith alone in Christ alone. Our faith then produces in us a desire to do good works that God has in fact prepared for us. But the Catholic view is that these works are essential. You have to have faith + merit. The primary way this done in their view is through the sacraments. These would include things like baptism (infant baptism in their view), confirmation, marriage, and communion among other things.
While many of these are practiced by us as protestants, they are not seen as sacraments necessary to add merit to our spiritual account. Perhaps the most problematic of these sacraments is the idea of penance. Penance is the act of performing works prescribed by a priest to reestablish a state of justification. In the Catholic view a person can commit a sin so bad that they lose their salvation. Penance would have to be done to get it back. This includes praying to Mary and many other acts which have no scriptural basis. In short, these practices (some of which are scriptural and some of which are not) are being done with the wrong motivation. God calls us to work for him, but not to earn our salvation. Our works should be the overflow of the work that God first did in us.