The difference between doctrines of faith and religious practice is the difference between thought and action. Doctrines of faith have only to do with what people believe. A person may believe in a thing or an idea without actually acting on that belief. For example, a person may believe that God approves of priests being married but that person may still not try to get the Catholic Church to change its teachings on that topic. Religious practice is about what people actually do. It is very possible for people to engage in religious practice without actually believing it. For example, a Catholic may take the sacrament of communion without actually believing that they are eating the literal flesh and drinking the literal blood of Jesus Christ.
The difference between these two is significant to our understanding of the Reformation because the Protestants were opposed both to Catholic religious practice and Catholic doctrines of faith. The difference between them helps us to think about the Reformation in a systematic way. It allows us to understand the different ways in which Protestants disagreed with the Catholic Church. For example, it helps us to understand that the Protestant opposition to infant baptism was opposition to a religious practice whereas opposition to the idea of justification by works was opposition to a doctrine of faith. This gives us a deeper and more organized understanding of the Reformation and the reasons behind it.