Beurre Monte' is a technique used to emulsify butter to prevent breaking at 180-190 F temperature. Butter is an emulsification of 80% milk fat, 18% water, and 2% milk solids. Heating butter above 160 degrees will cause it to "break" or separate into its different composition parts. By adding just 1 tbsp. of water it's possible to keep butter emulsified at a higher temperature.