Simple definition
A reusable block, now generally a synced pattern, shares the same content across multiple WordPress locations.
Purpose and use
Reusable block is the former name for shared block content now presented as a synced pattern.
How does it work?
It maintains one common source for a repeated callout or section instead of several independent copies.
Example and verification
When following older documentation, find the shared object among patterns and identify all locations using it.
Points to watch
Visually copying a section does not guarantee retention of its shared reference. Converting to ordinary blocks makes future edits independent.
In short
Understanding the terminology change helps locate the feature without confusing synchronization with duplication.