Example of how to alter the line item descriptions to include the ticket name and datetime names if the line item is not a promotion.

add_dates_to_line_item_desc.php

<?php //Please do not include the opening PHP tag if you already have one

function ee_revise_registration_checkout_line_item_desc(  $original_description, $line_item, $options ) {

  if( $line_item->ticket() instanceof EE_Ticket && $line_item->OBJ_type() !== 'Promotion' ) {
      $datetime_names = array();

      foreach( $line_item->ticket()->datetimes() as $datetime ) {
        $datetime_names[] = $datetime->get_dtt_display_name(true);
      }

      return sprintf( __( ' for "%1$s", <br />Date/time: %2$s', 'event_espresso' ),
        $line_item->ticket_event_name(),
        implode(", ", $datetime_names ) 
      );
  }
 
  return $original_description;
}
add_filter ('FHEE__EE_SPCO_Line_Item_Display_Strategy__item_row__desc', 'ee_revise_registration_checkout_line_item_desc', 10, 3);

add_dates_to_line_item_desc2.php

<?php //Please do not include the opening PHP tag if you already have one 

function ee_revise_registration_checkout_line_item_desc(  $original_description, $line_item, $options ) {
  
  if( $line_item->ticket() instanceof EE_Ticket && $line_item->OBJ_type() !== 'Promotion' ) {
      $datetime_dates = array();
      foreach( $line_item->ticket()->datetimes() as $datetime ) {
        $datetime_dates[] = $datetime->date_and_time_range();
      }
      return sprintf( __( ' for "%1$s", <br />Date/time: %2$s', 'event_espresso' ),
        $line_item->ticket_event_name(),
        implode(", ", $datetime_dates ) 
      );
  }
  return $original_description;
}
add_filter('FHEE__EE_SPCO_Line_Item_Display_Strategy__item_row__desc', 'ee_revise_registration_checkout_line_item_desc', 20, 3);