The Great Compromise (also known as the Connecticut Compromise) was a compromise between the Virginia Plan and the New Jersey Plan.
At the Constitutional Convention, there was a controversy over how seats in Congress should be apportioned. The small states wanted seats to be apportioned equally. They wanted each state to have the same amount of power in the Congress. This made sense for them because it would give them more power. New Jersey was a relatively small state and the New Jersey Plan proposed this sort of equal representation.
The large states, by contrast, wanted seats to be apportioned based on population. They felt that a state with more people in it ought to have more votes in Congress. Virginia was the biggest state in terms of population and so the Virginia Plan proposed representation based on population.
The Great Compromise set up a system in which there were two houses of Congress. Seats in the House of Representatives were to be apportioned by population. Seats in the Senate were to be apportioned on an equal basis with each state having two senators. This, of course, is the system that we now have.