Jude continues the nautical theme begun in verse 12 by calling the false ministers "raging waves of the sea." He describes them as storms in the church, causing trouble and turbulence wherever they go. James describes the doubting person in a similar way (James 1:6-8), as wind-tossed waves, double-minded, and unstable in everything. Such people will end up causing problems. Such waves toss people into hidden rocks, or as his brother Jude puts it, hidden reefs. Naive members can become caught in the turbulence and eventually be turned from the truth.
He then describes them as "foaming up their own shame." It is quite a picturesque phrase. He alludes to the foam on the beach after a storm. The strand is littered with all kinds of driftwood and other debris a storm can dredge up. They brag about their past feats as great accomplishments, but a godly eye sees them for what they are: shameful deeds.
He also calls them "wandering stars," another nautical allusion, this time to the movement of the planets. Mariners used the fixed stars - not the planets - to guide their ships over the trackless sea. They would align themselves toward a certain star to reach their destination. These teachers are supposed to be leaders, guides for those who are not as experienced on the road of life, but as we would say, they are all over the map! They go here and there, this way and that. It is the blind leading the blind, and anyone following them will fall into a ditch (Matthew 15:14). They are unreliable guides. They give horrible advice. They are not worth even talking to about one's problems because they will lead a person astray.
Jude foretells their fate at the end of the verse: "for whom is reserved the blackness of darkness forever." The literal translation of this is really dark: "Their fate is the utter darkness of darkness for eternity." Lights out forever! James 3:1 says that those who are teachers will receive the stricter judgment, and this is an example of it: the utter darkness of darkness forever. God takes the deception of His people personally.