The details type provides a collapsible area around other elements, similar to a fieldset. Fieldsets can only be used around form elements, whereas the details type can be used outside of them. Take a look at this example:

$form['name'] = array( 
  '#type' => 'details', 
  '#title' => $this->t('Name') 
); 
$form['name']['value'] = array( 
  '#type' => 'textfield', 
  '#title' => $this->t('Value'), 
);