The origin of the title is unknown. The motif of parallel triads on white keys, ornamented by black-key arpeggios, appearing from the beginning of the piece (measures 1-2), depicts a hazy, indistinct, and ambiguous scene, like a fog. In contrast, from measure 18, a syncopated rhythm with little movement appears (see musical example). This piece is structured by the contrast between these two motifs. From measure 24, the opening motif reappears, and through changes in dynamics and tempo, the density of the fog changes. From measure 38, the syncopated motif appears, and the two motifs intertwine, gradually fading away.

Musical Example: Syncopated Motif, Measures 18-21[1]

[1]  The score used in this commentary is as follows.

Claude Debussy. 1913.  Préludes. Livre II, Paris: Durand et Cie, D. & F. 8697, accessed 15 December 2019, International Scores Music Library Project, http://conquest.imslp.info/files/imglnks/usimg/4/48/IMSLP252535-PMLP02396-Debussy,_Claude-Preludes_2e_Livre_Durand_8697_filter.pdf.