A “man-of-war” was a term used by the British Royal Navy to refer to powerful warships from the 16th through the 19th century. The warship is blindly firing shells into the jungle, and Marlow sees this “incomprehensible, firing into a continent” as a metaphor for the larger concept of European colonization as being a futile attempt to control something that cannot and should not be controlled.
The word "arms” has somewhat of a double meaning here. In the previous passage, Kurtz is described body part by body part, so it is only natural to assume that these “arms” are his actual limbs. Conrad clarifies, though, that these “arms” are armaments, his guns and other weapons. This is related to the previous mention of Kurtz being “carved out of ivory”; there is no longer any separation between Kurtz and the work he engages in.
