Fry sauce is a condiment often served with French fries in many places in the world. Its usually a combination of one part ketchup and two parts mayo. There are many variations which add additional ingredients to the basic recipe.
Fry sauce, while its been around since the 1900’s was originally popularized in the US by a chef named Don Carlos Edwards who served it at his restaurant in Salt Lake City, Utah in the 1950’s.
Several fast food chains have individually branded variations of fry sauce and its also closely related to Yum Yum sauce which is popular in Japanese Steakhouses in the US.