Basics

FDM Filament that has had a Blowing Agent added to the filament
Typically it is heat activated, so heat can controll the foam ammount
A Relitively New Concept
Some "Wood" Filaments use this method to allow for a similar finish, and properties to wood (Althogh some do use actual cellulose)
Can allow for low density parts
MAY allow for 3D Printed Insulation if a Higher Temperature Plastic, and PERHAPS a higher temperature blowing agent
Also look into gas injection as an option

Foaming FDM Filament