understanding if statement [closed]

the code is executed when post type is post or page

No, only if the post type is page (because of the 'page' == $screen->id).

Because that if statement actually checks if the current admin screen is the one for editing or creating a Page (post type of page), where the URL looks like so: (note that the post type is not visible in the URL when a post/Page/CPT is being edited)

http://example.com/wp-admin/post.php?post=2&action=edit

• On such screens — editing or creating a post/Page/CPT, $screen->id is equivalent to $screen->post_type which is the post type (slug).

• And $screen->base is post which is the base name of the file name in the current URL — base name is the file name without the extension — so if the file name is post.php, the base name is post. However, when creating a post/Page/CPT, the file name is actually post-new.php, but still the $screen->base is post (and not post-new) because WordPress strips the -new part.

• Additionally, you can distinguish between editing and creating by checking the screen’s action; i.e. whether $screen->action is edit or add (e.g. 'edit' == $screen->action to check if the action is editing).

I need to add another custom post type (player) in that if
statement

Just change the:

'page' == $screen->id

to:

in_array( $screen->id, array( 'post', 'page', 'player' ) )

Or the full code:

if ( $screen &&                // 1. we've got a valid screen
    'post' == $screen->base && // 2. the screen base is either post.php or post-new.php
    // and 3. the post type is one of the array values.
    in_array( $screen->id, array( 'post', 'page', 'player' ) )
) {
    // your code here
}

I hope that helps, and you can check the get_current_screen()‘s function reference here. 🙂