Simple definition
A WordPress template part is a reusable component such as a site header or footer.
Purpose and use
A template part defines a shared structural component, such as a block theme's header or footer.
How does it work?
Multiple templates can reference the part, centralizing maintenance of its content and structure.
Example and verification
To establish the effect of a change, identify referencing templates and inspect their published output.
Points to watch
A template part and a synced pattern can both be shared, but their roles within theme structure differ.
In short
Template parts organize recurring site components without duplicating their definitions in every template.