Mercantilism was a commercial economic system that existed from the 16th to the 18th centuries. Mercantilism is founded on the idea that the world’s wealth is constant, thus many European countries tried to acquire the biggest possible part of it by boosting their exports and restricting their imports through tariffs.
Mercantilism is an economic theory and set of government polices based on the idea that gold is wealth and the nation should accumulate vast stores of gold so it can finance wars. Exports bring gold in, imports send gold out.
An historical note: All of the advocates for Mercantilism were import/export merchants (i.e., people who load up wagons and haul goods to far off lands) who, for the most part, were really tired being fleeced by foreign governments.