Sometimes you will need additional variables in your field templates. This is how you can add those.
Inside the Admin class, in the configureShowFields or configureFormFields methods:
$builder->add('fieldName', 'fieldType', [
'template' => 'your-template-file.html.twig',
'your_template_var' => $yourTemplateVar
]);
In the your-template-file.html.twig template:
{{ field_description.options.your_template_var }}