By either pressing and holding the Command and Space or clicking the icon on your Dock or Menu Bar, you can summon the same Siri you’ve grown accustomed to on your iPhone. However, this version of Siri is, in ways, much more powerful.
Beyond the standard query that might give you the weather or who’s up in the in playoff game, Siri on macOS Sierra can toggle system functions like Wi-Fi or Bluetooth or even pull up individuals files stored on your system based on what you tell Siri about the file.
Siri can even launch or close apps for you as well as store the results of your queries inside the Notifications area of the OS for later use. Just press Command+Space and get to asking.