Once we've taken heat out of the earth with the earth loop and squeezed it into higher temperatures with the geothermal heat pump, we must move it throughout your home. Your heat distribution system is how that is done, with air ducting or a hydronic system (radiant in-floor heating, baseboard radiators). Air ducting is the most common form of distribution in the USA.
It is extremely important for heat distribution systems to be sized and designed correctly, for your geothermal heat pump to operate efficiently. Most existing hydronic systems are designed for the higher temperatures of a fossil fuel boiler, and need reworked before they will function at all with the efficient, lower water-output temperatures of a water-to-water geothermal heat pump.
Your geothermal heat pump will be matched to the type of heat distribution system that you use (or vice versa): air ducting goes with a water-to-air geothermal heat pump, and hydronic heat distribution goes with a water-to-water geothermal heat pump.