Off mode means that Mysa won't be sending power to your heaters, essentially they wont be on. The thermostat will still be on but wont be turning your heaters on. This setting can be enabled on the thermostat itself or on the Mysa app.
On the thermostat: Hold down the down arrow until you see three dots on the display. Let go of the down arrow. The thermostat is now in off mode. Take a look at the video below to see how to do this.
In the Mysa app: Tap the tile of the thermostat you'd like to turn off. Press the icon in the bottom left corner, a pop up menu should appear. Tap off mode, represented by a plug.