What did Augustine mean when he said, "Now, as to follow the letter, and to take signs for the things that are signified by them, is a mark of weakness and bondage."
St Augustine is referring to the difference between realities and the signs we use to convey them. When these are confused, as happens with some interpretations of Sacred Scripture (this is the context of St Augustine's words here), then one takes the "letter" of Scripture without the "spirit" and so has a too-human, carnal interpretation of the word of God. St. Paul tells us, "The letter killeth, but the spirit giveth life." Thus the inauthentic, "literal" interpretation of the Bible is opposed to its true sense as revelation. Ultimately, St. Augustine resolves all questions of interpretation to love: if an interpretation serves charity and unity, then it is a permissible one; if it does not, and leads to sectarianism or schism, then the interpretation is false. This is his teaching in the books of his Confessions.