HSPF is an abbreviation for Heating Seasonal Performance Factor. It is the most commonly used measure of the heating efficiency of heat pumps. (The cooling efficiency of a heat pump is measured by its SEER.)
The important takeaway from this definition is that the HSPF is a seasonal measure (meaning that it takes into account the fact that the heat pumps rarely for as long as is optimal during Spring and Fall). A heat pump with a high HSPF is more efficient than a heat pump with a low HSPF.
New heat pumps manufactured after Jan 1, 2015 are required to have an HSPF of at least 8.2. The most efficient heat pumps have an HSPF of 13.