Over a period of about 400 years, about 10-15 million African people were kidnapped and sold into slavery. These people were packed onto to crowded ships, and brought to the New World, the Americans, as a source of free labor. People were traded for goods.
The slave trade was profitable and cruel. People would return from working in the fields or from hunting, and find their families missing. In some cases, entire villages were captured by the slave traders and loaded onto ships.
Some kingdoms, like Benin, refused to participate in the slave trade. But others were eager to trade African people for goods like beads and textiles and brandy and horses and guns. The European traders had no trouble filling ships headed for the New World with captured people.
Many people died on the trip to the New World. People were packed on board without any thought of their comfort or even survival. There were no bathroom facilities. There was very little food. It's no wonder disease killed so many.
After the Civil War, it was illegal to buy slaves. But slaves continued to be traded in Central and South American for another 40 years.