Glycerine is a sugar alcohol and is commonly used in candymaking and some patisserie elements; it keeps sugars from solidifying. It is edible; if you read labels, you'll find it in a lot of foods! And if you read labels, you will find it in many skincare products as well.

In this recipe, it is probably acting to stabilize the dressing and help smooth the texture, just guessing (gelatin is used for similar reasons to stabilize mousses and whipped cream).