Adaptive brightness is a feature in Windows that uses a light sensor (if one is installed on your device) to automatically adjust the display brightness to match the lighting conditions in your computer's surroundings.
Sometimes, in Windows 10, the brightness won't adjust automatically after a reboot.
An communication error occurs with the hardware while the system is booting.
Restarting the device again should get adaptive brightness working again.
For more information on adaptive brightness (ALS) see this MSDN page.