This is a filter as used in WP.
The first argument of the filter is that value that you can adjust and return. In this case $args is an array of values that will be used when ACF calls get_terms(). Other values passed to your filter are for information. $field is an array with all of the setting of the current field that the filter is being run for and $post_id is the post ID of the current post being edited.
I don’t know why the filter is not working for your field. Can you give me more information about the field? Is this taxonomy field a sub field of another field?
