Definition
Wet stacking is the accumulation of unburned fuel and carbon in the exhaust and combustion spaces of a diesel run for long periods at very light load, caused by combustion temperatures staying below the level needed to burn fuel completely.
A diesel is designed to run warm and worked. At very light loads it runs cooler than intended, fuel does not burn completely, and the residue accumulates — in the exhaust, around the valves, and as glazing on the cylinder bores.
Where it turns up
Two classic cases. The yacht that idles at anchor to charge batteries, and the generator sized to a worst case that never happens, running for hours at a fraction of its rating. Both produce smoking, fouled engines long before their hours suggest.
See can I run a diesel at idle to charge batteries and how to size a marine generator.
What to do instead
Size generators to the real load. Charge with an adequate alternator, solar and a properly sized bank rather than idling. Where light-load running is unavoidable, follow it with a period at proper load. Severe glazing is a workshop problem, not something a hard run will clear.