Part of the biblical commandment of wearing tzitzit (a four-cornered fringed garment) is to attach a blue thread to each corner (Numbers 15:38). Techeilet refers to the dye used to make the threads and derives from an ancient Semitic word for purple shell. Techeilet was manufactured from the chilazon, which in modern Hebrew means a snail but in ancient Hebrew might have a referred to a variety of shellfish.
According to the Midrash, the rebellious Korach posed the sophistical question whether a cloak made entirely of techeilet would be required to have a thread of the same material on each corner.
The implication was that it was absurd that a small thread would be necessary for compliance with the law. So too Moses, one individual, was unnecessary to lead an entire nation of holy people. Thus today, the expression Tallit shekulo techeilet (a cloak that is entirely of techeilet) refers to falsely pious people.
With the destruction of the Second Temple, the source of techeilet became lost and people began wearing white tzitzit and doing without techeilet. Rabbi Gershon Hanoch Leiner (1839-1891), known as the Ruzhiner Rebbe, travelled across Europe to identify the original chilazon and techeilet and in Italy found the cuttlefish to fit all the criteria. He supervised the manufacture of techeilet and to this day one finds people wearing tzitzit with the blue thread dyed from this fish.