Good question! The Rachio controller is compatible with 24V AC solenoid valves which is the standard for U.S. irrigation systems. So if you're located in North America then your irrigation system is most likely compatible with a Rachio Smart Sprinkler Controller.
If your system does not use these kinds of valves you will want to replace them with 24V AC valves before installing your controller. If you are unsure of what kind of valves you have then you may wish to consult a local irrigation professional for help in determining this.