ilustri

The trap here is a genuinely clever one. Esperanto has a busy suffix -il- that names the tool for a job: tranĉi to cut gives tranĉilo a knife, haki to chop gives hakilo an axe, kombi gives kombilo a comb. And the language really does have a root lustr-: a lustro is a chandelier. So ilustri invites the reading il- plus lustri, something like to do the light-fitting.

Two things kill that reading. First, -il- is a suffix, which in Esperanto means it comes after the root, never before it. There is no prefix il- in the language at all. Second, the root here is ilustr-, borrowed entire from Latin illustrare, and it happens to share an ancestor with lustro several centuries upstream without being built from it. The resemblance is real etymology and useless morphology, which is the most dangerous combination a learner meets.

Two senses, and the second one carries the weight. The literal sense is putting pictures into a text: an ilustrita libro is an illustrated book and an ilustraĵo is a single illustration, with -aĵ- doing its usual job of turning an action into the concrete thing it produces. The figurative sense is illuminating a point with an ekzemplo, and it is the one you will write more often.

Close relatives worth keeping apart.Montri is to show, plainly and without interpretation. Desegni is to draw, the physical act. Klarigi is to explain, which works on the reader's understanding rather than on the page. Ilustri sits between them: you make something clearer by putting a picture, real or rhetorical, beside it.