This is a 10m roll of 1mm thick double-sided foam tape, the same kind of tape you'd use to stick a number plate to a car. It's available in two different widths - 6mm or 12mm (select above).
It's great for securing things inside your project enclosures, attaching electronics to robot chassis, sticking panels together and hundreds of other handy uses.
The thickness and 'squishiness' of the foam allows for imperfections in the joining surfaces too, so it's great for PCBs and other things which don't always have a perfectly smooth surface.
No maker bits box is complete without a roll of double-sided foam tape!