What is the main idea of mercantilism?

The main idea of Mercantilism is that a nation’s wealth  and power were best served by increasing exports and so involved increasing trade. It was an economic principle. It was an economic policy followed by European colonial empires. 

Thus such policies frequently led to war and motivated colonial expansion. Mercantilist theory varies in sophistication from one writer to another and has evolved over time.

For further reading check the following articles:

• Portuguese India
• British Raj
• Dutch Colonialism in India