WordPress posts by default have an excerpt meta box where you can manually type in a custom excerpt.

Pages don’t have this feature out-of-the-box.

If you built a site that mainly uses pages (as opposed to posts), or perhaps for any other reason, you may want to be able to add custom excerpts to your WordPress pages just like you can with posts.

To enable excerpts on pages, add this code to your functions.php file:

add_post_type_support( 'page', 'excerpt' );

Now when you go create a new page or open an existing page for editing, there should be an excerpt meta box where you can type your custom excerpt.

Looks something like this: