The macroscopic explanation is this: as the money supply increases, the money supply curve moves to the right, intersecting with the money demand curve at a new point, at which point the interest rate decreases.

It can also be understood this way: supply increases, currency supply exceeds demand, and the extra currency is used to purchase bonds, bond prices rise, and interest rates fall.

Extended information:

The LM curve is a curve used to describe the relationship between national income and interest rates in the equilibrium state of the money market.

The LM curve represents the trajectory of various combinations of income and interest rates in the money market when money supply equals money demand. The mathematical expression of the LM curve is M/P=KY-HR, and its slope is positive.

The LM curve is a curve described by different combinations of income and equilibrium interest rates that keep the money market in equilibrium. In other words, on the LM curve, each point represents a combination of income and interest rates. These combination points just make the money market in equilibrium.

Reference materials:

Baidu Encyclopedia-LM Curve