It was a treaty between Portugal and Spain in 1494 in which they agreed to divide up all the land in the Americas between the two of them, no matter who was already living there. (I know, really stupid!) They divided the New World by drawing a line in the Atlantic Ocean, All lands east of that line were claimed by Portugal. All lands west of that line were claimed by Spain.
This is why most Latin American nations are Spanish-speaking countries, for instance, but Portuguese is the leading official language in Brazil, because Portugal took Brazil and Spain took most of the other nations.
