Drupal provides a mechanism to alter database SELECT queries before they are executed. Contributed and custom modules may use this mechanism to restrict access to certain entities by implementing hook_query_alter() or hook_query_TAG_alter() in order to add additional conditions. Queries can be distinguished by means of query tags. As the documentation on EntityFieldQuery::addTag() suggests, access-tags on entity queries normally follow the form ENTITY_TYPE_access (e.g. node_access). However, the taxonomy module's access query tag predated this system and used term_access as the query tag instead of taxonomy_term_access.
As a result, before this security release modules wishing to restrict access to taxonomy terms may have implemented an unsupported tag, or needed to look for both tags (term_access and taxonomy_term_access) in order to be compatible with queries generated both by Drupal core as well as those generated by contributed modules like Entity Reference. Otherwise information on taxonomy terms might have been disclosed to unprivileged users.