During setup, your Google Nest thermostat senses what wires are connected and asks you a few questions so it can offer different features specifically for your heating or cooling system.
Most thermostats are designed for conventional forced air systems, so they treat every system the same, even though modern high efficiency heating and cooling systems work differently. A high efficiency system simply won't be as efficient or comfortable if it's treated like a conventional system.
System Match changes the Nest thermostat's learning algorithms and activates different features depending on what kind of system you have to help reduce wear on your system and improve temperature accuracy. These features, including Early-On, Heat Pump Balance, True Radiant and Filter Reminders, are designed to make your heating or cooling system work better so you can save energy and stay more comfortable.