Simple definition
A WordPress block theme uses block-based templates and supports customization through the Site Editor.
Purpose and use
A block theme defines templates and site parts primarily through blocks usable in the Site Editor.
How does it work?
Templates structure page types and global styles coordinate presentation. Theme files and saved customizations both contribute to output.
Example and verification
Before switching themes, inspect custom templates, patterns and dependencies carrying essential site elements.
Points to watch
A block theme does not remove every use of PHP or plugins. A saved customization can also override a change in theme files.
In short
The theme supplies an editable structure whose variants and overrides matter for maintenance.