One of the defining characteristics of classical Protestantism in all its forms is sola scriptura, which in Latin means “Scripture alone” or “only Scripture.” This can sometimes be misunderstood as the Bible being absolutely the only authority Christians should look to on any matter, but while that is held by some Christians, it is not what is meant. What sola scriptura means is that Scripture is the only infallible rule of faith or authority, not that it is the only one of any kind. In other words, those who hold to sola scriptura do not have a problem with turning to philosophy, history, natural science, tradition, or even experience to help inform their beliefs on a variety of matters, but they simply do not treat these other sources of knowledge or authority as infallible.
This view contrasts with that of the Roman Catholic Church (obviously, since Luther was reacting to Rome). The official teaching of the RCC of course affirms that Scripture itself is infallible, but it also holds that Sacred Tradition, the tradition allegedly handed down from the Apostles, is also infallible. There may be a bit of variation on how Catholics may rank Tradition vs. Scripture, but it’s enough for now to state that official Catholic teaching upholds both the Bible and Sacred Tradition as infallible. A good example of how this plays out is in the doctrines of Mary. It is considered dogma, for instance, that Mary’s body was physically assumed to heaven, yet even Catholic theologians will admit that there is basically nothing in the Bible that even hints at this. What they will claim is that this is nonetheless infallibly true because tradition says so, even though no early church father even mentions this as well. Protestants are often aghast that Catholics could call such a teaching dogma when there is no written evidence of it from the Bible, from the early church fathers, and from early history about Christianity, but Catholics hold fast to what they believe was passed on in tradition.