I'm going to assume this is some form of mayonnaise until told otherwise.
It's fake mayonnaise essentially. From Wikipedia: "Current primary ingredients are water, soybean oil, vinegar, sugar, modified corn starch, eggs. Ingredients making up less than 2% of product include salt, mustard flour, paprika, spice, natural flavor, potassium sorbate, enzyme modified egg yolk, and dried garlic."
Indeed it is.
Local view: The sort of bread I've seen people abroad use for toast sandwich, is essentially what we call "toast" in Germany too. Specifically, "toast", in Germany, refers to mostly pre-cut, soft bread with a squarish cross section so bland in taste and so deprived of nutrient content that only toasting will turn it into "food". Nutrient content is just as absent, after toasting, but at least the burned parts help digestion. :]
So here, a toast sandwich is a slice of (toasted) bread between two untoasted slices of the same bread, more or less. I've never seen any native eat sth. like this, in Germany.
On the other hand, we do have gustatorially misguided contemporaries here, too, that do think that mayonnaise-derived stuff belongs on a salad.