A small prayer case holds Scripture passages. Pious Jews wear it at prayer. At prayer, Orthodox Jewish males wear two small, black leather boxes. They contain Scripture.
The phylactery was probably not a box of Scripture. It was a strip of parchment with four Old Testament passages in Hebrew. The passages were:
The Deuteronomy 6:4–9 passage contains the “Shema”—the confession of God being one Lord. All four passages say that God commands his people to bind his laws on their hands and have them as "frontlets" between their eyes. Jews interpreted this in a non-literal sense and forgo physical adornment. Some Jews took the command literally. They began to wear portions of their Scriptures on their foreheads and hands. Exactly when they began to do this is not agreed upon by scholars. There is an explicit mention of the practice as early as 100 BC in a Jewish nonbiblical document. Some believe it began as early as the fourth century BC, if not earlier.
In Matthew 23:5, Jesus condemned the scribes and Pharisees for, among other things, their habit to "broaden their phylacteries." The passage's context is Jesus's rejection of their explicit religious practices. Apparently, the broad phylactery would impress others with how religious the wearer was. It was evidence of pride, pretense, and hypocrisy in religion.