Answer:
The part-time job given to Jabez Wilson by the Red-Headed League, namely to copy out the Encyclopaedia Britannica, was a complete waste of time. The criminals' aim in all of this was only to get him out of his pawnshop from 10:00 to 14:00 each weekday.
While Wilson was occupied in copying out all things concerned with the letter A, the criminals dug a tunnel connecting his cellar to the cellar of the City and Suburban Bank, in order to steal "30,000 napoleons from the Bank of France" that were being kept in the cellar.
The main criminal was John Clay, "the murderer, thief, smasher, and forger." His accomplice was the man calling himself Duncan Ross who recruited Jabez Wilson. Ross had bright red hair, as did Wilson, which is what first suggested the idea of the Red-Headed League to the criminals. John Clay, calling himself Vincent Spaulding, got himself hired as Wilson's assistant in order to manipulate him into taking the job.
Wilson only consulted Sherlock Holmes after his employment abruptly ended and the Red-Headed League dissolved (when the tunnel was complete). He wanted to track them down, hoping to keep his high-paying job. Holmes worked out the real seriousness of the situation and laid a trap to catch the criminals in the act of breaking into the bank through the cellar floor.