There's the real problem and then there's the anxiety or worry associated with it. The saying makes an analogy to suggest that the real problem is like the principal amount of a debt and the anxiety or worry is like interest on the debt (an added amount or burden over and above the original debt). It also comments that you have to suffer the anxiety or worry before the "debt" is due. That is, before people actually have to confront and resolve a problem, they are already "paying" by suffering from anxiety and worry about the problem. So... the underlying point seems to be "don't worry about problems that haven't come to pass yet". Worry about them when they actually occur, not before they occur.

It means that there is no point worrying about something that may not happen in the future.