INTEREST is the cost of money. So yeah whenever you use money (that i not yours) then you pay interest. If you let others use your money (lend it) then you get paid interest.
How does interest control inflation or how does it stimulate the economy you ask? Well let's first make things clear that the central bank can print as much money as it deems right (although I would add my opinion here that they are printing too much) and lend it out to bank and the banks to the consumer or businesses. If there is TOO MUCH MONEY chasing the same or worse lessening goods in the economy then prices rise. For example I have 5 loaves of bread and we have P10 in total money supply then the bread would cost P2. Now if I printed more money and made the supply at P20 then the bread will now cost P4 thus inflation. I know its a very simplistic example, but it works something like that. Now if they raise interest rates they slow down the printing of money cause less people will borrow because the cost is now higher. If interest rates are low more people would like to borrow (thus more money supply in the economy) because the cost of borrowing is lower. Tightening interest rates slows the economy since there is less money circulating around, loosening the interest rates though makes money very accessible and that stimulates business expansion thus the economy strengthens.
Mahirap ma-explain but I hope na-gets mo more or less...
Here is a video from the European Central Bank which should give beginners an idea how inflation and deflation is controlled via monetary policy...
YouTube - European Central Bank Educational Video