A treatise is usually a long and drawn out exposition of a subject in written form. It is longer than an essay and tends to give very deep consideration to a particular subject. The most famous treatise of all is probably The Art of War by Sun Tzu with a close second being The Prince by Niccolò Machiavelli.
Being able to correctly pronounce this word will give you the appearance of being a person who might also spend his evenings deeply engrossed in a treatise or two. So you would do well to learn it if you wish to appear to be at the acme of intelligent society. The word is said: “tree-tiss” with the emphasis on the “tree”.