Fiber conduit pipe (its proper name) was "made of ground cellulose (wood) fibres bound together with a special water resistant adhesive, and, thereafter, impregnated with liquefied coal tar pitch," according to SewerHistory.org.
Lightweight, it was easy for plumbers to carry. Brittle, it was easy for them to cut with regular carpenter's saws.
But like today's PVC and ABC pipe, it had a tendency to collapse. Thus, the pipe had to be properly bedded in sand and pea gravel to reduce stress on the pipe.