The touchpad is a tactile zone that allows you to activate Dreem features without going through the application. To learn all about the possible interactions with the touchpad, please read "Understanding the Touchpad.
When you start your night, the touchpad will lock, which means you will not be able to activate it inadvertently.
To unlock it during the night, perform a "Swipe", ie a lateral movement of your finger on the touchpad, to the left or to the right. You can then use it to activate a sleep technique or stop your night for example.