This is a cascade-protection notice, mostly intended for near the top of the documentation pages of template that are subject to cascaded protection because of their transclusion in some other page, and which are expected to have that protection indefinitely.
While a similar template is automatically generated by the system from MediaWiki:Cascadeprotected as an editnotice when someone tries to edit ("view source") on such a template, they have to actually try that before they get any inkling that the template is protected because of cascading. By putting a notice also on the template's /doc page, this should forestall attempts to have the template's protection level reduced to template-editor level (such requests must always be declined in cases of cascade-protection).
Usage
Just add {{Cascade-protected template}} near the top of the template's /doc subpage (usually right below the {{documentation subpage}} header).
In the odd case that the cascade-protected page is not a template, |page= can be used to change the word, e.g. |page=interface page.
This template is not for temporary protection. Many pages get short-term cascade-protected because of transclusion from an article listed on the Main Page for a few days. We do not need to tag then untag 100+ templates with this notice during that period. Volunteer editorial time is Wikipedia's most precious resource, and WP is not BureaucracyPedia.