Interest, the cost of money
Capital in a free market is allocated via the price system. The interest rate is the price to borrow money as debt. When capital is raised as equity, investors expect to receive dividends and capital gains. The sum of these two sources of capital is the cost of equity.
The four key factors that control the supply/demand for capital are:
Production Opportunity
The returns possible from the investment in productive assets, are production opportunities
Time Preferences for Consumption
Consumer preference for saving verses consuming
Risk
The chance the invest will not return as expected
Inflation
The amount by which prices increase / time