Marlow and his men find a note which reads "Wood for you. Hurry up. Approach cautiously" (Conrad, Ch. 2) on a stack of firewood at the hut fifty miles below the inner-station.
How does this note serve to advance the plot?
The note functions as a threat and makes the audience unsure as to whether Marlow and his crew will continue their journey.
The note functions as a type of foreshadowing. The audience is made aware that Marlow and his crew need to hurry to the station as something is wrong. The mood becomes suspenseful because the note implies some danger is approaching.
The note creates ambiguity. The audience does not understand the note (nor does Marlow). Therefore, it is unclear as to how Marlow's crew will proceed.
The note functions as a flashback for Marlow. At this point, the novella returns to the frame of the Nellie as Marlow remembers the old women "guarding" the office prior to his departure.
The note helps develop Marlow's character. Even though there is a warning on the note, Marlow chooses to continue obsessing over Kurtz.