In chapter 13, Keynes explains how the rate of interest enters into the complex of other factors comprising the general theory. He covers how interest rate is effectively the “price of debt” (Chapter 13, Section IV) and how it varies according to factors such as the liquidity-preference in the aggregate (Chapter 13, Section II). He also says that preference for liquidity is mostly due to the uncertainty around future interest rates (Chapter 13, Section II).
Lastly, he also touches on how rates of investment depend on the relationship between the marginal efficiency of capital and the interest rate. That said, I still do not adequately understand this concept of the schedule of the marginal efficiency of capital.