What is mercantilism in the colonies?

The thing to keep in mind about mercantilism is that it represented an economic theory concerning the nature of trade, and the State's role and interest in controlling it. That being said, it certainly played a critical role in colonization. In fact, colonization was very closely tied with mercantilist ideas, as a means by which European Powers sought to maintain an economic advantage over their rivals, and maintain a favorable balance of trade.
From this perspective, mercantilism underlines the entire colonial project. England used its colonial subjects as trading partners. If you were to move beyond the Thirteen Colonies, you would find in the Caribbean a particularly lucrative location, thanks to the high demand of sugar back in Europe. Similarly, in North America, you can point towards cash crops such as tobacco and cotton, which became well known cornerstones of the southern economies. Additionally, the advantages need not be strictly economic; colonies could also be used as a means of acquiring and/or protecting key strategic and military resources. For example, consider New England's shipbuilding industry, which provided ships for the Royal Navy. Finally, consider the Navigation Acts, which placed restrictions and regulations on colonial commerce. To conclude, mercantilism was a cornerstone of colonization, embedded across the entire history.