To illuminate something is to light it up so it can be seen clearly. It can also mean to explain an idea in a way that makes it easier to understand.
From Latin "illuminare" meaning "to light up" or "make bright," from "in-" (on) and "lumen" (light). The figurative sense of "enlighten the mind" grew naturally from the physical idea of shining light.
Our brains use light as a metaphor for understanding so deeply that we say "I see" when we actually mean "I understand." To illuminate a topic is to flip on a mental light switch, turning vague shadows into sharp shapes.
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