Prerequisite: intermediate macro.
I reckon some people might be getting those two things muddled. The difference matters. Actually, the difference is the only thing that matters. It's the spread between those two interest rates, not the levels of those two interest rates, that matters for Aggregate Demand.
Simplify massively. There are two rates of interest. There is the rate of interest you get paid for holding money. Call it Rm. And there is the rate of interest you pay/get paid for borrowing/lending money. Call it Rb. Those are two conceptually very different interest rates. Rm is the rate of interest you get paid for the media of exchange in your pocket or chequing account. Rb is the rate of interest you get paid for the bonds or non-media of exchange IOUs you own.
[Update: I have a new post that adds a picture to this one.]