The Teller Amendment was a law that said that the US would not take Cuba as an American territory if it were to win the Spanish-American War. This was meant to be a statement of America's lack of desire for empire. However, it was more or less overturned by the Platt Amendment after the war.
When the US was debating whether to fight Spain over the issue of Cuba, there was a segment of the US that was strongly against taking an empire. These people disliked the idea of empire for various reasons, ranging from idealism (America should not deny other countries their independence) to racism (America did not want to have a whole bunch of non-white people added to its territory). The Teller Amendment was a statement of this attitude.
However, by the end of the war, the imperialists had won out. This was shown in the Platt Amendment, which essentially gave the US control of Cuba even though Cuba was officially independent.