The meaning is quite literal (especially compared to some idiom!). It can be, and usually is, used in a literal sense. When you finish your meal your host could say 'have some more, there's plenty more where that came from' - meaning that there is plenty more food - probably in the pot, or in the oven. Or perhaps if you 'borrowed' something like a pen from a colleague - they could say 'any time, there are plenty more where that came from'. It can be used figuratively as well though. Someone has been very rude, and you have thoroughly pummeled them. They are lying in the gutter, and look up at you with an evil look. You could say 'got any more smart comments? There's plenty more where that came from' -meaning that you are willing to hit them again, if they make another rude remark. (Not that I condone violence, of course! )