To understand means a few things, but they all essentially boil down to having a correct (or at least correct enough to be useful for your usecase(s)) model for

To understand means a few things, but they all essentially boil down to having a correct (or at least correct enough to be useful for your usecase(s)) model for something in your head. Have you told someone something like "no, don't do it that way because <insert non-obvious downstream problem>, instead, do <insert alternative strategy that achieves better outcomes>"? That's an artifact of understanding and of the model you've developed for that thing. It's well described as a mixture of knowledge and wisdom, and is essentially the property of knowing the effect that pulling a lever will cause, coupled with good judgement about how, when, and why to pull the lever.