Often, it is very useful to put movement limitations on the bones of a rig, for several reasons—usually, to make them easier to work with, but also to establish a maximum range for the rotation of the limbs or other parts like in the mandible or the eyelids.
Two types of limits for the bones are: by the Transform locks, and by bone constraints.
Let's start with the Transform locks:
Select the eyebrow.L bone and if not already present, press the N key to call the 3D viewport Properties sidepanel. Go to the Transform subpanel, which is the first entry at the top, or also to the Transform Locks subpanel under the Bone window in the main Properties panel to the right of the screen:
The Transform subpanel in the N Properties sidepanel and the corresponding Transform Locks subpanel under the main Properties panel