“…only just as I began to think of getting out of her way, as you would for a somnambulist…” (pg. 9)
A somnambulist is simply a sleepwalker. Sleep walking occurs when someone is falling asleep but has not reached REM (rapid eye movement) sleep. They are not aware of their actions and will often not realize they are a sleepwalker until someone wakes up them while they are in the act. Sleepwalking is usually attributed to high levels of stress in the sleepwalkers.
EDIT: Conrad uses somnambulism as a kind of metaphor in Heart of Darkness. It could be said that Marlow (at one point) and many of the Company members (especially the pilgrims) are all sleepwalking-- they're going through the motions of life but seem to have no emotion or real depth to them.