I am impressed that you are studying the American revolution and its causes.
The idea of "representation" here is that of a representative in the legislative body in Parliament. It is a principle of the English system of gov't (as I understand it as an American who inherited many aspects of it) that all taxes are to be raised by the legislature (and not the King). The people retain the right and the responsibility to ensure the funding of the gov't that they decide to have.
During he time of the American colonies, the people who lived in America did not have representation in the British Parliament. They had no vote that made a difference in how the British gov't conducted itself and raised revenue. But they were taxed by this gov't.
This was the major cause (as we understand it today) of the revolution. The revolution started because the American colonists wanted their rights as British subjects, including the right to be represented in Parliament. There were also issue of the mercantile system and how the colonies were only supposed to supply raw materials to the mother country and buy all finished products from her.