The Torah commands us to include a blue ("techelet") fringe among the tzitzit fringes. This fringe must be dyed blue with the blood of a shellfish called "chilazon." This rare fish is a denizen of the Mediterranean Sea.
After the Jewish people were exiled from the Holy Land, the chilazon was no longer available. For many centuries Jews wore tzitzit without a techelet fringe.
In the past century, there have been certain venerable rabbis who claimed to have identified the chilazon, based on this fish's distinguishing features described in the Talmud. Since then, tzitzit with techelet has made a comeback, and one can often spot people sporting such tallitot. There are many, however, who have cast doubts on the re-identification of the chilazon.
Rabbi Sholom DovBer, the fifth Rebbe of Lubavitch, maintained that according to the teachings of Kabbalah the chilazon will not reemerge until the coming of the Messiah.