What was the Boston Tea Party in response to?
The Tea Act, basically a tax.
The French and Indian War (which was the American theater of Britain's Seven Year War with France) was very expensive; George III essentially won it by outspending his enemies. He intended to pay for it by taxing his American colonists, reasoning that they should pay for their own ongoing protection. Some of these taxes were in the form of Stamp Acts, expensive stamps required for official documents and transactions. One of these, the Tea Act, was instituted in 1773, a decade after the end of the war it was ostensibly paying for.
The colonists resented all the taxes, of course, and this was simply one more in a series. Since the colonists had no representation in Parliament, there was some question of why they were still paying these taxes at all, much less at an ever-increasing rate.
The Boston Tea Party wasn't so much a formal protest as a rowdy lout rampage. The participants, Boston's Sons of Liberty, dressed up as Indians and threw the East India Company's entire shipboard cargo of tea into Boston Harbor on Dec. 16, 1773. Protests in other American colonies simply refused to let the ships unload the cargo, but this one actually destroyed it.
Parliament responded with harsh measures that popularized the cause of the pro-independence movement, ultimately resulting in the American Revolution.