The three sources I checked all defined wetting as "The phenomenon whereby a liquid filler metal or flux spreads and adheres in a thin continuous layer on a solid base metal". I'm just curious, how does the poster describe wetting? A cold roll (overlap) situation?

When a bead (puddle) "wets in" it makes a smooth transition from filler to base material with minimal under cutting or cold lap.

In other words all parameters (volts, amps & speed) are correct.

The examples I use to illustrate 'wetting' is to think of drops of water on the hood of a car. If the paint is beat up and has not been waxed for a long time, the water will wet out. If the wax is fresh, the water will bead up and not wet out very well.
Moltel metal from welding acts in similar manner. If the metal does not wet out well, there is a good chance of lack of fusion, overlap, and other problems. The puddle needs to flow where you want it to go.

You could get into surface tension and all that goes with that subject, but this should give you the 'nuts n bolts'.
Chet Guilford