WordPress at a glance
action is not described

enqueue_block_editor_assets action-hook . WP 5.0.0

Fires after block assets have been enqueued for the editing interface.

Call add_action on any hook before 'admin_enqueue_scripts'.

In the function call you supply, simply use wp_enqueue_script and wp_enqueue_style to add your functionality to the block editor.

Usage

add_action( 'enqueue_block_editor_assets', 'action_function_name_9235' );
function action_function_name_9235(){
	// action...
}

Where the hook is called

In file: /wp-admin/edit-form-blocks.php
enqueue_block_editor_assets
wp-admin/edit-form-blocks.php 357
do_action( 'enqueue_block_editor_assets' );

Where the hook is used (in WP core)

wp-includes/default-filters.php 486
add_action( 'enqueue_block_editor_assets', 'wp_enqueue_registered_block_scripts_and_styles' );