Like all other webmail services and email programs, Yahoo Mail includes by default a "Trash" folder: you may know that special email folder and functionality in other applications and services either as "Recycle Bin", "Deleted Items" folder, etc. The Trash folder in your Yahoo Mail account never contains any email message unless and until you manually delete one or more messages contained in other folders. When you delete an email, it is not permanently deleted right away - instead, Yahoo Mail moves it to the Trash to give you a few days to restore and recover any emails accidentally deleted - but, to keep its millions of web users from accumulating deleted messages, Yahoo will automatically empty your trash in a process called "purging". Once Yahoo Mail purges your trash, there is no chance to recover an email deleted before that point - think of the Trash folder as a "last chance" folder.
Like other email folders in Yahoo Mail, you can right away tell how many messages are inside the Trash folder: the number shown between brackets, if any, indicates the number of unread messages inside the Trash folder. And if you move your mouse pointer (cursor) and hover above the Trash, as shown on the screenshot below, Yahoo Mail will show you the total number of deleted emails - both Read and Unread.
As mentioned earlier, the Trash is automatically purged - and since Yahoo Mail gives you plenty of storage space, this means that you do not need to manually remove deleted emails from the Trash. But if you want to, just click on the "Empty" link displayed next to the Trash icon and label on the left - as shown by the tooltip, this will permanently remove these deleted emails from your Yahoo Mail account, without any chance of recovery.
Yahoo Mail will show a final confirmation message before proceeding to purge the messages currently inside the Trash: "Are you sure you want to permanently delete the contents of the Trash folder?" Click "OK" to delete them, or "Cancel" to keep them in the Trash:
Yahoo Mail Tip: note that the "Empty" link is always showing on the right of the Trash folder, whether it currently contains deleted emails or not (same applies to the Spam folder as well). In doubt, hover above the Trash icon, and you'll see a "Trash contains 0 emails" tooltip: