Albert Barnes Commentary Numbers 15:38

"Speak unto the children of Israel, and bid them that they make them fringes in the borders of their garments throughout their generations, and that they put upon the fringe of each border a cord of blue:" — Numbers 15:38 (ASV)

That they put upon the fringe of the borders a ribband of blue - Reader, that they add to the fringes of the borders (or corners) a thread of blue (compare the marginal references).

These fringes are considered to be of Egyptian origin. The ordinary outer Jewish garment was a quadrangular piece of cloth like a modern plaid, to the corners of which, in conformity with this command, a tassel was attached.

Each tassel had a conspicuous thread of deep blue, this color being doubtless symbolic of the heavenly origin of the commandments of which it was to serve as a memento. Tradition determined that the other threads should be white—this color being an emblem of purity .

The arrangement of the threads and knots, to which the Jews attached the greatest importance, was so adjusted as to set forth symbolically the 613 precepts of which the Law was believed to consist. In our Lord’s time, the Pharisees enlarged their fringes (Matthew 23:5) in order to obtain reputation for their piety.

In later times however, the Jews have worn the fringed garment (tālı̂̄th) of a smaller size and as an under-dress. Its use is still retained, especially at morning prayer in the Synagogue.