"Apt to teach" describes someone who is prone or inclined to lecture people

1 Timothy 3:2 lists "apt to teach" among the qualifications for an overseer, from the Greek "didaktikos," meaning skillful in teaching. "Apt" comes from Latin "aptus," fitted or suitable, the same root behind "adapt" and "aptitude." So the phrase is a qualification of competence: this person is equipped and able to teach well.

Modern English mostly uses "apt" in the sense of "likely" or "prone to" ("apt to forget," "apt to fail"), which flips the meaning from a positive skill into a mere tendency, as if Paul just meant someone who talks a lot, rather than someone actually qualified to instruct.