Simple definition
A WordPress action hook executes registered callbacks when a defined event occurs.
Purpose and use
A WordPress action runs a function at a defined point in core, theme or plugin behavior.
How does it work?
Registered functions run according to priority and receive the arguments supplied by that extension point.
Example and verification
For unexpectedly repeated processing, inspect hooks, priorities and safeguards against unintended repeated execution.
Points to watch
An action does not automatically provide authorization or input validation required by the added operation.
In short
The hook defines when to intervene; the callback remains responsible for its operation's effects and controls.