More info:

After the Boston tea party the British colonies decisively pushed back in no uncertain terms, the British parliament, in turn retaliated by passing legislation, backed by the threat of lethal force, against the 13 colonies called (I kid you not!) the Intolerable Acts aka the Coercive Acts.
There was a lot of tit for tat retaliation by both sides but eventually the British authorities got their arses kicked. Some Americans began boycotting the drinking of tea and to this day coffee is the preferred beverage.

The rattle snake with it’s 13 rattles signified the unity of the 13 colonies when the revolutionary war broke out against the British authorities. 

The Yankies have been celebrating this period ever since, they have museums, songs, films, political movements about it, for example in the 2000’s a libertarian political party was formed called the Boston Tea Party.  Later on, on an anniversary of the original Boston Tea Party a “money bomb” political fundraiser “Tea Party” for presidential candidate Ron Paul broke records raising millions on the day!

Now days the flag implies strong willed warning to aggressors that defenders will be vigilant and defend against coercion or aggression, that’s to say pushing back hard against immoral or arbitrary authoritarianism world wide.