Matrix multiplication is just one of those things that is not intuitive – at least not at first. You have just had so many years of multiplication meaning one thing and then you have this entirely new definition to work with! It certainly takes some getting used to. (and if you continue to study advanced math – get used to that idea of “getting used to” things)
The easiest way to think of it is “rows hit columns and fill up rows”. This process is repeated until you run out of rows in the first matrix. This is seen in the following animation (no sound).
You can read more about how to multiply matrices here, including examples and important properties: Multiplying matrices.