What do you understand by the drain of Indian wealth during the colonial period?

The most important feature of India’s foreign trade throughout the colonial period was generation of large export surplus. But this caused scarcity in the domestic market. Further, this export surplus did not result in any row of gold or silver into India. Rather, this was used to make payments for the expenses incurred by,  expenses on war  office setup of the British government and  Import of invisible items.  All these led to the drain of Indian wealth during the colonial period.