Since this question asks what Thomas Jefferson did “to” America and not what he did “for” America, I would say that the best answer is that Jefferson more than doubled the size of the United States.
If we asked what Jefferson did for America, we would be more likely to look at things like his role in writing the Declaration of Independence or the things that he did as the third president of the country. However, this question asks what he did “to” America. The best answer for that is that Jefferson made the country much larger. He did this through the Louisiana Purchase.
The Louisiana Purchase was made in 1803. In that year, the US (with Jefferson as president) agreed to buy over 800,000 square miles of land from France. It paid $15 million for the territory. The goal of the purchase was to enlarge the US but, more importantly, to make sure that the US would have control over the Mississippi River, which would guarantee that it would be able to use that river to transport goods out to the sea through New Orleans. This is the main thing that Thomas Jefferson did “to” the United States.