Most central air systems operate at one speed. It turns on to get to the temperature you set on the thermostat and turns off once it’s met. When the system is off, the temperature in your home will naturally rise or fall, and the system will continue to cycle on and off to maintain the set temperature.
A variable speed system operates at different speeds depending on the need for heating or cooling. This modulation allows the system to run at a higher speed when necessary to meet the set temperature quickly and then automatically slows down to maintain it. Because it’s not continually turning on and off, it uses less energy. So while an 18 SEER central air conditioner has the same rating as an 18 SEER variable speed air conditioner, it’s actually less efficient over time.