Understand seems like a pretty straightforward English word. It comes directly from Old English, it’s composed of two simpler words, under and stand, and it’s had its current meaning, to comprehend, since our earliest records of it. Still, it’s something of an etymological mystery. What does “standing under” have to do with understanding?
Most of our metaphors for understanding have to do with getting, grasping, or taking. When we understand we “get it,” “catch the drift” or “get a handle on it.” The root of comprehend is the Latin prehendere, grasp. Perceive comes from capere, “take hold of.” Many languages refer to these metaphors in their vocabulary of understanding.
But the “standing under” metaphor doesn’t seem to show up anywhere else. In the languages most closely related to English, the stand idea does come into play, but the words for understand have a different metaphor at work. German verstehen, Dutch verstaan, and Scandinavian forstå are all related the Old English word, forstandan, which meant either “stand in front of” or “stand away/apart from.” This spatial arrangement has ties to another common metaphor that relates being able to see something with understanding (as in I see, or speculate, from Latin for “look at”) and to another that relates understanding to the idea of separating from (discern, from Latin for “separate”).
Scholars have been arguing about the motivations for Germanic verbs of understanding for a long time now and plenty of explanations have been proposed. One holds that the under- prefix also meant between or among, and to understand was to stand between things in order to separate and discern among them. Another holds that Old English forstandan got mixed up with no-longer surviving synonyms for understand like undergetan (under get), underniman (under take), and underthencan (under think), but in that case it’s still not clear how those other words used under to relate to the idea of comprehension.
We may never know what the original coiners of understand had in mind, but that hasn't stopped us from making good use of it for over 1000 years. We get it anyway.