Source

File: wp-admin/includes/meta-boxes.php

function post_excerpt_meta_box( $post ) {
	?>
<label class="screen-reader-text" for="excerpt"><?php _e( 'Excerpt' ); ?></label><textarea rows="1" cols="40" name="excerpt" id="excerpt"><?php echo $post->post_excerpt; // textarea_escaped ?></textarea>
<p>
	<?php
	printf(
		/* translators: %s: Documentation URL. */
		__( 'Excerpts are optional hand-crafted summaries of your content that can be used in your theme. <a href="%s">Learn more about manual excerpts</a>.' ),
		__( 'https://wordpress.org/support/article/excerpt/' )
	);
	?>
</p>
	<?php
}