The wildcard SSL option lets you secure as many subdomains as you like, in addition to your main domain name.
For instance, you could secure https://mail.yourdomain.com, https://secure.yourdomain.com, https://news.yourdomain.com ... and as many others as you want, all for a single extra payment.
Wildcard SSL does cost extra, but that's because it can save you money. Previously you'd have had to buy a separate certificate for each subdomain. Now you only need one.