Z-axis tape is made by 3M, and is used to bond two conductive surfaces together and allow a small (we'd say keep it under 100mA) current to flow through the tape. But here's the cool part: its only conductive across the thickness of the tape itself, not along the length or width. Mindblowing, right? We took a macro shot of the tape above so you can see how it works. Its a regular adhesive tape with little chunks of conductive material every 0.1mm or so. The adhesive does the bonding, the little chunks do the conducting.
For a quick fun project we put some tape on a Gemma and Neopixels: remove the white cover first, then stick the circuit board onto the brown paper+tape. Cut out the shape around the circuit board, then carefully peel off the brown paper. Repeat for the neopixels. Make traces on a piece of paper and stick the Gemma and Neopixels down: ta-dah, paper circuit!
Each order comes with a strip of 50 x 150mm tape. Its big enough you can use it for a bunch of projects. Just cut out the size you need and save the rest.