This aphorism is very popular among "liturgically-minded" Christians, who rely on it to emphasize the importance of liturgy and (sometimes) to justify their interest in liturgical minutiae. And as an Anglo-Catholic "bells and smells" man from the beginning, it is an important principle for me as well. (I would note in passing that the aphorism is, if anything, more important for "liturgical Protestants" (Anglo-Catholics and Lutherans) than for Catholics and Orthodox. For Catholics and (especially) Orthodox, the centrality of the liturgy is simply a given, not something that must be continually argued for and justified.)
So what does this old saw actually mean?
For many the meaning is simply that the liturgy is important because the liturgy expresses what we believe; that we ought to be careful about how we worship because of what it says about our faith. Sloppy and irreverent worship reveals sloppy theological thinking, and heretical worship certainly indicates heretical doctrine. A stronger form of this attitude says that we must edit our forms of worship to conform to correct theology. We are to sit at our editor's desk with our liturgical texts at one hand and our confessional documents and systematic theology textbook at the other hand, wielding the editor's pen to ensure that the former conforms strictly to the latter.
For others it means that liturgical texts serve as a source-book for theologizing. The texts themselves are a doctrinal authority; subordinate to the Scriptures, perhaps, but an authority just the same. The extreme form of this attitude would picture the same editor's desk as I did in the last paragraph, only this time we are to conform the confessions to the liturgical texts, not the other way around.
I think there is some truth in both of the views I have just outlined. But I think the most important word in the saying is the one that is usually left out when it is quoted. Usually we say simply "lex orandi, lex credendi", as if to say "there's some relationship between belief and prayer, but we're not saying exactly what it is." But the aphorism is Lex Orandi Lex EST Credendi -- the rule of what is to be prayed IS the rule of what is to be believed. It is not a statement of the way things ought to be, but a statement of the way things are.
And in fact it is in our worship that we are taught, and receive, and take to heart the faith that we hold. Our Lutheran Fathers express that clearly when they note (AC V) that "that we may obtain this faith, the ministry of teaching the Gospel and administering the sacraments was instituted." They did not say that we obtain our faith by reading the Bible, going to Sunday School, or even at our mother's knee. It is through the Church's ministry of Word and Sacrament that we are given the faith that saves us -- and that means liturgically. All other ways by which we learn our faith are (at best) extensions of and secondary to what happens in the liturgical assembly. Because extra-liturgical ways of appropriating our faith have no divine promise attached to them -- unlike the liturgical ministry of Word and Sacrament, they are not covenanted means of grace.
A Church body may say that its rule of faith is found in its confessional books; but the faith which is actually imparted to the folks in the pews is what they hear and experience when they are actually sitting in those pews: lex orandi lex est credendi.