SDSL means Synchronous Digital Subscriber Line compared to the more typical broadband services known as ADSL - Asynchronous Digital Subsciber Line.

The difference being the speed at which information can be uploaded and downloaded.  Typically ADSL services enable downloading at between 512k and 8mb, but restrict uploading to between 448k and 832k.  SDSL enables up to 2mb in both directions, so for site with high "send" requirements, such as digital artwork, large database files etc., an SDSL service is the preferred solution.