What does the popular DON'T TREAD ON ME picture with the snake respresent?

Historically, it represented a warning to England not to oppress the colonies; that if England tried to 'step' on the colonies then the colonies would 'bite' back. In contemporary usage, it's used as a warning to the government not to oppress the people.

This is the Gadsen flag
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https://en.wikipedia.org/wiki/Gadsden_flag

Represents the Americsn revolution asnd the founding fathers.
It was the first battle flag of the colonial militia when they fought England