What is Interest rate(s)?

When we refer to interest rates on macroeconomics books what are we exactly talking about in real financial markets? There are thousands of interest rates people can choose from when they invest their money and they differ massively one to the other. Thanks.
Update: As you asked I try to add some details to my original question: when we have models showing the choice between keeping the money in bank accounts and investing it and the where the decision is based on investment rates, what ir are we referring to? Bonds? Stock markets? A pondered average?
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