何謂加息、減息？

In theory, when there is an exogenous change in money supply (institutional factor), interest rate wll change and thus change national income.

But in reality, most central banks use the interest rate as its policy instrument aims at holding the interest rate constant at a target level by adjusting the MS.

But even the interest rate is held constant, monetary policies can still be carried out by central banks effectively, for instance, central bank can raise (reduce) the interest rate level to cool down (stimulate) the economy.

Central banks choose to control interest rate directly instead of money supply may be because of the relative transaction costs involve, they are uncertain about the effect on interest rate if they change money supply only.

It means the effect of changing in Money Supply.

For example, if there is a rise money supply along with the constant money demand, the money aggregate availability in the market is abundant that the cost for lending or borrowing or finding money to use is lower, and that cost is the interest paid to lender or received by the borrower. Thus the interest rate falls by the increment of Money Supply, vice versa.

I hope this brief explaning can help you understand this concept.