BTU, which stands for British Thermal Unit, is a measurement of energy used in the heating and cooling industry to quantify the heat output or capacity of a furnace or other heating devices. In the context of a furnace, the BTU rating indicates how much heat the furnace can generate per hour. A higher BTU rating means the furnace can produce more heat and is often used to match the heating capacity of the furnace to the size and heating needs of a building. It's important to select a furnace with an appropriate BTU rating for your space, as one that is too small may struggle to heat your home adequately, while one that is too large can lead to energy wastage and discomfort. Proper sizing and efficiency considerations are crucial when choosing a furnace for optimal heating performance and efficiency.