Stands for "Symmetric Digital Subscriber Line." SDSL is a type of of DSL, which is used for transferring data over copper telephone lines. The "symmetric" part of the term means that an SDSL connection has the same maximum upload and download speeds.
ADSL, on the other hand, typically provides much faster download speeds than upload speeds. Because most Internet users download much more data than they upload, ISPs usually offer ADSL connections rather than SDSL.