Do not brag or else you will fall. Here "fall" or "taken down" is figurative. It might refer to losing status, position, reputation, or respect.

a similar expression in English is "pride goeth before a fall" - goeth is an old-fashioned, poetic way of saying 'goes'

Hey Nada.. how are you? The verb ''boast'' means to ''speak in exaggerated or excessively proud terms of one's possessions, skills, or superior qualities.'' In addition, the word ''lest'' means, in this case, the same as ''in order to avoid that...'' So far as that is concerned, the sentence ''Do not boast lest you be taken down'' conveys the idea that ''you should not speak highly of yourself so that you (or perhaps your chances for success) are not ruinied or destroyed. Did you get the idea? Hope that helps!!!