Beurre manie (pronounced "burr man-YAY") is a mixture of equal parts flour and butter kneaded together and then whisked into a sauce or soup to thicken it.
Beurre manie is a quick alternative to making a roux, which involves melting clarified butter, whisking in flour and then briefly cooking it. Beurre manie is made with whole butter rather than clarified butter.
The term beurre manie means "kneaded butter" in French.
Also see: What is a Liaison?