The Great Compromise (also known as the Connecticut Compromise) was created in response to an argument about how to apportion seats in the national legislature. This was an argument between the large states and the small states.
When the men at the Constitutional Convention were trying to create what we now call the Congress, they had to decide how many representatives each state would have. The large states wanted the representation to be based on population. They wanted to have more representatives than the small states did because they had more people than those states. The small states wanted representation to be equal. They felt that each state should be equal to each other state, regardless of population.
The Great Compromise created a bicameral legislature that was a mix of each of these ideas. It created the House of Representatives, where states with more population get more representatives. It also created the Senate where all states are equally represented regardless of size.
This compromise, then, came about because of an argument about how to decide how many representatives each state would have in the Congress.