Flag of the United Colonies, 1776; and flag of the United States, 1776-1777.
The canton shows the flag of Britain that had been in use since 1606 (see here: Flag Britain 1606-1801) and had thirteen stripes representing the thirteen colonies of the United Colonies and later the thirteen states of the United States.
This was the flag adopted by the United Colonies (in full the United Colonies of North America, also known then and prior as the Thirteen Colonies), a territorial entity then still within the British Empire that was demanding that Britain acknowledge self-government in the United Colonies including recognizing the demands by its leaders that no taxation should be imposed upon the people of the United Colonies without their acceptance of it through a decision of a representative assembly. This was the beginning of the American Revolution. The dispute between the British and American governments was not resolved, and months after the United Colonies was founded it moved to independence becoming the United States of America.
Note 1: I use a standard palette of shades of colours for the flag designs. Contrary to the images of flags depicted on Wikipedia, I do not follow the "official" shades of colours claimed by governments as well as alternate history flags based on flags of existing countries. The problem is that it commonly results in bizarre shading of a flag if they are taken literally - in practice such "official" colour shades are almost never applied and standard reds, blues, etc, are used.