In May 1773, England passed the Tea Act, requiring American colonists to purchase any and all tea from the British-operated (and struggling) East India Company.
Three ships importing tea were docked in Boston when colonists refused to unload the tea from the boats, and asked for it to be taken to England.
It's important to remember that colonists' interests were not represented in British Parliament. For that reason, and in protest of the high and unfair taxation on tea, 116 participants dumped 342 containers of tea (some 90,000 pounds) in the Boston harbor on December 16th, 1773.
This historical event was one of many which expressed growing resentment among Colonial leadership, precipitating the American Revolution.