I would argue that there were three main achievements of Congress under the Articles of Confederation. Two of them had to do with just getting the United States going as a country and a third had to do with expanding the country.
Perhaps the most important achievement of Congress under the Articles was the fact that it managed to fund, and in some ways direct, a successful rebellion against England. This was, of course, a very difficult task. The Congress had to work to keep together a country that was just getting started while finding ways to pay for a war. Without this achievement, there would have been no United States.
After the war was over, the work was not done. The colonies had not had much of a sense that they constituted a single country and the Congress had to make sure the new states did not break apart and form their own separate countries. While the Articles of Confederation created a Congress that was perhaps too weak, a stronger Congress might have driven some states to form their own countries. The Congress managed to keep the country together after the war.
One way in which they kept the country together (while also allowing it to expand) was through the passage of the Northwest Ordinances. Before the war, various colonies claimed lands in what was then called the Northwest. This was in areas like Ohio and Indiana. This could have caused conflict between the various colonies. In the Northwest Ordinances, Congress defused this tension by saying that those territories would not belong to any colony. Instead, they would be admitted as separate states when they got populations big enough to warrant that step. This helped prevent conflict between the existing states and it set up a way to create new states when the time was right.