{parent} stands inside a {block} of a page that overrides a block of its layout or wrapper. It is replaced by the content of the block being overridden, so the page can add to a region - before it, after it, or around it - instead of replacing it entirely.

It is resolved in the text while the page is assembled, together with the blocks and {extends}, and is gone before the page renders. Through a chain of layouts, each {parent} is the next block down the chain.

A {parent} anywhere else - outside a block, or in a block that overrides nothing - reaches the render, and is an error under the strict check.