This is adhesive backed copper ribbon mainly used for stained glass projects. It can also be used for decorative edging and can even be used for electronics! You use it by wrapping around the edges of glass or other materials and then soldering the joins. It is known as copper foil technique and also Tiffany Technique after its American inventor.
This tape has a black back instead of the usual copper back. This is for when you want to blacken the solder with Patina but are using light coloured glass or mirror an don’t want the back copper to show up.
3M Venturetape and EDCO copper foil tapes come in 33m rolls. ARCHS copper foil comes in 50m rolls which represents great value