The food dictionary at Epicurious.com gives the following definition:
French for “kneaded butter”, beurre manié is a paste made of softened butter and flour (usually in equal parts) that is used to thicken sauces.
Beurre manié serves the dual purpose of thickening as well as making your sauce nice and glossy at the same time! An easy way to blend it in is to grate it into your dish. Come autumn when I will be cooking more dishes requiring thickening this is going to be a staple of my freezer.(Image courtesy of The Cook’s Thesaurus)