In WordPress Trash post is a post that has been deleted from your WordPress site and moved to the trash/recycle bin.
Normally trash posts are not visible at front end in your site and periodically must be deleted/cleaned-up.
There are multiple plugin available like Advanced Database Cleaner to clean-up these trash posts.
Also you can manually execute the following SQL query to clean-up trash posts. Execute following query in your phpMyAdmin query editor:
DELETE FROM wp_posts WHERE post_status = 'trash';
