What is investment? How is it related to national saving?

Investments in an economy means the total expenditure made by public and private sector on economic activities, to create infrastructure, businesses, employment.

Domestic savings directly finance the investments in an economy and if there is a gap between savings and investment then that gap is financed by fiscal deficit or current account deficit, it can be explained by the following equation

(G - T) + (M - X) = I - S

Fiscal deficit + current account deficit = savings investment gap

FD + CAD = SIG