A heat pump is a viable option to heat your pool...if you are patient.
Whereas a gas-powered heater uses gas (either natural gas or propane) to heat the water, and an electric heater uses electricity to heat the water, a heat pump uses refrigerant technology to capture heat from the outside air, intensify (raise) this heat, and then transfer this heat to the pool water. Since a heat pump utilizes that actual outside (ambient) air temperature, proper placement of the heat pump on the equipment pad is key so that the heat pump has the best access to air.
Since a heat pump generates it heat from the outside air (which is free for all of us to breathe and use) and since the process of heating this outside air is cyclical, a heat pump is the most economical and efficient way to heat water. The heat pump itself, though, is fairly expensive - more so than a gas-powered heater. But, this upfront expense is not an actual expense; it is an investment, because once the heat pump is purchased, the monthly operating expenses are very low, which allows you to recuperate your upfront money and see a return on your investment while the heat pump is still fairly new. And, heat pumps will last longer than a traditional gas-powered heater (of course, assuming both are maintained and cared for properly).
If your pool currently does not have a method of heating and you are looking for a method to heat your pool, keep in mind that even though a gas-powered heater (the unit itself, regardless if it is for natural gas or propane) will cost less than a heat pump, you will have other costs to consider. For a natural gas heater, you will have to pay a licensed plumber a fee to supply a gas line and plumb that gas line from the gas meter (or an approved supply of natural gas) to the heater, which will be fairly expensive. And for a propane heater, you will have to buy (or lease) a propane tank and pay labor to have the propane tank delivered, installed, and initially filled as well as a fee to a licensed plumber a fee to supply a gas line and plumb that gas line from the propane tank to the heater, which will also be fairly expensive. Therefore, if you are in the market for a new heat source, even though a heat pump (the unit itself) will cost more than a gas-powered heater (either natural gas or propane), the extra costs associated with the gas-powered heater will make the cost of the heat pump more attractive. And again, the monthly cost for a heat pump is always significantly lower than any other heating method.
Obviously, the nicer the day (in terms of warmth of the temperature), the better the heat pump will work.
Like gas-powered heaters, the number of BTUs of the heat pump is the main criteria that is used to properly-size a heat pump. The acronym BTU stands for British Thermal Unit. It is not important for you to know and understand all of the science and math behind the acronym BTU; just know that the listed BTUs of the heat pump is the main criteria for properly sizing a heat pump. There is a formula to properly size a heat pump.
Because heat pumps are not manufactured with high BTU ratings and outputs, larger pools will likely need two heat pumps (plumbed and manifolded correctly) to efficiently heat the water.
You can read the Pool Manual for additional information about pool heat pumps as well as the formula to properly size a heat pump.