At 3.8.3, there is a button on the "Attributes Toolbar" that is always greyed out. When I hover the cursor over it, the response is
"No action selected":
The QGIS documentation makes no mention of this button. What is its purpose?
As the documentation on the QGIS Website this is the Actions identify tool
First you need to set an action to perform:
Here is some actions setup (via right click on the layer properties):
Here is a result of using the actions with the 'FeatureID' selected