The haze in the glow

From Part 1, ¶ 9:

The yarns of seamen have a direct simplicity, the whole meaning of which lies within the shell of a cracked nut. But Marlow was not typical (if his propensity to spin yarns be excepted), and to him the meaning of an episode was not inside like a kernel but outside, enveloping the tale which brought it out only as a glow brings out a haze, in the likeness of one of these misty halos that sometimes are made visible by the spectral illumination of moonshine.

From Part 1, ¶ 14:

We looked on, waiting patiently—there was nothing else to do till the end of the flood; but it was only after a long silence, when he said, in a hesitating voice, “I suppose you fellows remember I did once turn fresh-water sailor for a bit,” that we knew we were fated, before the ebb began to run, to hear about one of Marlow’s inconclusive experiences.

In the narrator’s indirect way (Marlow’s experiences are “inconclusive,” the meaning of his yarn is “not inside” but “outside”) he—with the author—is warning his audience about the tale Marlow will relate.

[And if you're confused, note that an unnamed narrator starts our tale, then turns it over to Marlow for most of the duration.]

What do his warnings lead us to expect, and not to expect, from the tale Marlow tells? Can we assign exact meaning and interpretation to utterances like, "The horror! The horror!"?

Remember—your response to this is one paragraph, tops!