A conductive 3D printing filament is any filament that conduct electricity, allowing the user to print anything from circuit boards to sensors to wearable electronics. Conductive 3D printing filaments are usually compounds of PLA, a dispersant (something used to spread small particles) and some form of conductive Carbon, like graphene.
When selecting a conductive filament there is another parameter you must consider in addition to the usual properties used to choose a filament. Volume Resistivity is the measure of a materials resistance to electricity within a cubic centimetre given in the units Ohm-cm. The lower the volume resistivity, the better the material will conduct electricity.
Conductive filaments are ideal for projects that require low-voltage circuitry, touch screen interaction or touch sensitivity. Conductive 3D printing filaments based on PLA tend to be a little more flexible than normal PLA but may also have lower z-axis adhesion. They generally print with low warp so a heated bed is not strictly necessary but may improve results.