Gemara Ta’anit (30a-b) describes the ideal pre-Tisha B’av meal as eaten while sitting in solitude on the floor, in the dingiest section of the house, with nothing more than a small jug of water and a dry crust of bread. Rema records a custom to dip the bread in ashes. Other halachot teach that one may not have more than one cooked dish, should not formally sit down to eat, nor join with friends to incur an obligation towards zimun. Further, no meat or wine should be consumed at this meal. (This Talmudic baseline prohibiting meat and wine was extended in Ashkenaz to cover the nine days.)
If this sounds like the negative imprint of Shabbat and Yom Tov, that is the point. On these celebratory days, halacha expressly encourages eating meat and drinking wine, meals with multiple courses, inviting friends to formally sit at the table, and concluding with zimun. The laws of Tisha B’Av assume this is the norm and then teach the opposite. The stark contrast of dipping the hard bread in ash underscores how some of the most poignant messages of the fast are found not in the fasting but davka in the eating.