The Manila galleon trade is probably more significant in the history of the world as a whole than it is in the history of the United States, but it does have significance for both. The Manila galleon trade contributed to what was arguably the first truly globalized trade network in history. The trade was significant for the history of the United States because it helped bring about the development of California.
In order to understand these impacts, let us first look at what the Manila galleon trade was. This was trade between Manila, in the Philippines Islands, and Acapulco, on the west coast of Mexico. Both the Philippines and Mexico were colonies of Spain.
The Manila galleon trade began in 1565. The Spanish had ruled the Philippines and Mexico for a few decades by that time, but they had not yet found a way to sail east across the Pacific from the Philippines to Mexico. In 1565, they found a region in which winds blew to the east, allowing their sailing ships to go across the Pacific in that direction.
Once this happened, a global trade was created. This was the first time that there had been direct contact between Asia and the Americas. It was not possible for goods to from the Americas to Asia or Europe, from Europe to Asia or the Americas, and from Asia to either the Americas or Europe. This meant that essentially all of the world was tied together to at least some degree by trade. This was a momentous event in world history.
The Manila galleon trade was not as important to the history of the United States. Instead, its connection to US history is somewhat tangential. The winds that blew east across the Pacific often caused the galleons to reach the Americas off the coast of California. The Spanish eventually explored California in search of places that they could allow the galleons to land and refit themselves after the long voyage. This helped to develop California to some small degree. It ensured that the Spanish would have control of California and that their control would pass to Mexico when it became independent. The US would then take California from Mexico in the Mexican-American War.