In the 1400s and 1500s, as you have read, Europeans set up small forts on the coast of West Africa in order to resupply their ships and profit from local trade, especially in gold. As Europeans built colonies in the Americas, they needed large numbers of laborers to make their colonies profitable. By the 1600s, they increasingly turned to Africa to provide that labor.
This 1800s diagram shows how slaves were so tightly crammed in small spaces that they had to lie side by side with little room to move for many hours at a time.
Slavery has existed all over the world since ancient times: ancient Egypt, Greece, and Rome, as well as China, Persia, the Aztecs, and other societies had enslaved people. The English word slave comes from Slav, the people of Eastern Europe who were often sold into slavery in the Middle Ages.
The Arabs also used slave labor. Some were captives taken from Africa.
In the Middle East, enslaved Africans worked on large farming estates or large-scale irrigation projects. Others became artisans, soldiers, or merchants.
In the 1400s and early 1500s, the Portuguese and other Europeans brought a few Africans back to Europe as slaves. There, Africans were seen as exotic servants of the rich. As European colonies in the Americas grew, however, Europeans turned to slave laborers to clear plantations, or the large estates run by an owner or an owner's overseer.