Unless I'm mistaken, “s” refers to a FIFO socket. See the FIFO(4) manual page for more details.
Basically, a FIFO refers to a small in-memory buffer that one process can write to while another one reads from. The processes are controlled by SIGSTART/SIGSTOP (I think) so that you can never write to a full buffer or read from an empty one.
You can create a FIFO socket or “named pipe” using the command:
Hope that helps,
— Robert J. Lee