A few hundred years ago, the rich people wanted spices. Spices did not grow in Europe, so they had to be brought from Asia. People made a route to go from Europe to Asia to buy spices. The route was very difficult and so the spices were expensive. People tried to find ways cheaper than the spice route, like going the other way around the world. This is how many things were discovered, like the Western Hemisphere.
The spice trade was where countries traded spices for gold and other things. They did this so they could get new spices or gain money and gold by selling their own. Sometimes people traded the spices because their country is running out of that certain spice.