"Wet on wet" is a technique rather than a type of paint. It is often used on the final primer or colour coats to achieve a high build smooth finish and minimise polishing or sanding. One coat of paint is sprayed, then the panel is immediately sprayed again while the first coat is still wet. Still a good risk of sags and drips , but much less so than if you tried to get the same quantity of paint on in one go. Takes a lot of practice to avoid sags, so I tend to apply single coats and rely on sanding or polishing for finishing.
Sorry not quite right
Wet on wet primer is a primer that is used and requires no sanding, just apply the paint straight on top, genrally for new panels i.e.
prep panel 320 d.a.
appy one or two coats of wet on wet primer allow to dry for 5 mins then follow with top coat.
The method malcom mentioned has another term most commonly known as a "double header" where two coats of paint are applied without and time left between coats.
Most commonly used back when cellulose was widly used.