"Fritten" are French fried potatoes, term is derived from the French 'pommes frites'. Germans refer to their favorite fast food stall as a "Frittenbude" (Bude = hut, informal).
You have a choice to get your French fries (= Pommes, Fritten) with either ketchup or mayonaise. "Pommes mit Majo" (pronounced "maajoh") means the latter. Note: both ketchup and mayonaise may cost you something extra.