Boost is a countdown timer. When you add Boost, the device or Group it was applied to will toggle the opposite state, wait for the countdown to finish the toggle back to the original state.
For example: if your device is switched on and you Boost it for 5 minutes, the device will switch off and in 5 minutes time your device will switch on. Likewise, if your device is switched off and you Boost it for 15 minutes, the device will switch on and in 15 minutes time your device will switch off.
To add Boost to a device, head to the 'Devices' page (navigation drawer > Devices) and select the device you would like to Boost.
To add Boost to a Group, head to the 'Groups' page (navigation drawer > Groups) and select the Group you would like to Boost.
Select the Boost button (rocket icon between edit and delete in top right corner).
Select how long you would like the Boost to last then press 'Add Boost'.