It's a standardized format for transferring recipies. If you have some sort of recipe software that read that format, it lets you import the recipe into your collection.

There's a little info here:

http://en.wikipedia.org/wiki/RecipeML

and here:

http://www.formatdata.com/recipeml/

And one (free) program that uses it here:

http://www.recipewebservice.com/largorecipes/

My hope is that someday you'll be able to send the RecipeML file to your magic cooking robot and it will just make the dish for you.... I guess when that day comes, we'll be ready :-)