Short answers, pulled from the story.
Investment in macroeconomics is defined as the additions made to a nation's capital stock of buildings, equipment, software, and inventories during a year. It is also described as spending on productive physical capital, such as machinery and construction of buildings, plus changes to inventories, counted as part of total spending on goods and services.
Investment in macroeconomics includes residential investment in housing, non-residential fixed investment in things like machinery or factories, human capital investment in workforce education, and inventory investment, the accumulation of goods inventories, whether intentional or unintentional.
Gross investment is a component of a country's total output, alongside consumption, government spending, and net exports. Net investment deducts depreciation from that gross figure, and net fixed investment measures the net increase in the capital stock over a year.
A higher interest rate negatively affects investment in macroeconomics because it raises the cost of acquiring the funds needed to purchase investment goods. Higher income has the opposite effect, positively affecting investment because it signals greater opportunities to sell what capital can produce.
Tobin's q is the ratio between a physical asset's market value and its replacement value, and investment in macroeconomics is modeled as an increasing function of that ratio. When the ratio is greater than one, machinery bought at one price can generate output worth a larger market value, producing positive economic profit.
Investment in macroeconomics is a flow, since it is expenditure measured over a period of time such as a year. Capital, by contrast, is a stock, representing accumulated net investment up to a point in time.