Content before this block will be shown in the excerpt on your archives page.
Block example
This is some content before the core/more block.
This is some content after the core/more block.
Code structure
On the page itself
<span id="more-421"></span>The number after more is the post ID.
On the archive page
Content on the archive page depends on the active theme and on the length of the content before the core/more block.