Andalouse sauce (sauce andalouse) is a mayonnaise-based condiment flavored with tomato paste and sweet or roasted red peppers, along with seasonings such as paprika, cayenne, and garlic. Despite its name evoking Spain's Andalusia region, it isn't a traditional Spanish or Portuguese sauce; its earliest documented form appears in early 20th-century French cooking (it's in Escoffier's 1903 "Le Guide Culinaire"), and it later became especially popular in Belgium, where it's a standard dip for fries, and it remains widely used in France and Luxembourg as well.
The sauce is made by whisking mayonnaise with tomato paste and finely chopped or puréed peppers, then seasoning to taste; it's typically chilled briefly to let the flavors meld. Beyond its role as a fries dip, it's used as a topping for grilled meats, fish, and vegetables, and as a sandwich spread or salad dressing.
Recipes vary in spiciness and in which peppers are used, but the mayonnaise-tomato-pepper base is consistent across versions.