Realex Payments are making changes to the web certificates. If you use Realex Payments to process credit cards on your website and you a 128 bit (sha1) SSL certificate then you need to upgrade this certificate to 256 bit (sha2). If you are using Realex’s hosted page i.e. where your customer processes the payment on the Realex page (and not on your website) then this is not an issue for you.
An SSL is a secure socket layer which is typically displayed as https:// on the address bar of a website using it.