Phylacteries are boxes made of animal skin (commonly leather) that is worn on the head and on the arm/hand area. They include four verses written on parchment, all of which remind them of the solemn covenant to God and their duties towards him.
The four verses are: Exodus 13:1-10, Exodus 13:11-14, Deuteronomy 6:4-9, and Deuteronomy 11:13-21. (If you desire to look up the exact text for these verses, there is a search bar in the side column).

The head phylactery has four compartments, one for each verse. The hand/arm phylactery has only one compartment, with all four verses written on one piece of paper.

The basis of this practice is found in all four verses, but can be summed up as such: “And it will be like a sign on your hand and a symbol on your forehead that the Lord brought us out of Egypt with his mighty hand.” (Exodus 13:16).