See C. O. Sauer, The Early Spanish Main (1966); A. O. Exquemelin, The Buccaneers and Marooners of America (1891, repr. 1971).
The Spanish Main was the mainland coast of the Spanish Empire around the Caribbean. The mainland of which was initially called "Spanish America" (thus, "Spanish Main"), it included Florida, Mexico, Central America and the north coast of South America. In time it became a general term for the seaways around the Spanish possessions in the Caribbean.
Major loading ports were Cartagena de Indias in New Granada, Porto Bello on the Isthmus of Darien and Veracruz in New Spain (with wares brought by the Manila Galleons transported overland from Acapulco), and from there they were shipped to Spain by the famous Spanish treasure fleets. This made the Spanish Main a haunt of pirates and privateers, and gave the name a notorious and romantic allure.