A wet belt is a timing belt that is constantly wet with oil from the sump. Silly silly idea that should never have been put into production, but Ford have been doing it for years on a few of their engines (as well as some other Euro brands like Renault etc). The 1.0L Ecoboost engine is probably the most "well known" engine for wet belt issues, as it was fitted to a bunch of cheap cars like the Fiesta and Focus - so heaps of them are on the roads in Europe.
Basically the rubber on the timing belt starts breaking down over time and falls off in chunks and blocks the oil pickup in the sump. So if the timing belt doesn't snap from degradation, then your engine will lose oil pressure and kill itself that way instead.
So if you are unlucky enough to own an engine with a wet belt, do oil changes more often than recommended by the manufacturer, and do the timing belt service more often than recommended to avoid a very expensive failure.
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