It's great for that DC adapter you found with the too-short cable. Plug this 3' extension in, and you're good to go.
So you finished your project, and found the right power supply in your junk bin, but the dang thing is just...too...SHORT. GRRRR. Either you extend your AC to the adapter (expensive, cumbersome), snip and extend the wire on the power supply (cheap, time-consuming), OR use one of these super-handy 2.1mm barrel jack extension cords.
One end has the male connector, the other has the female, in 2.1mm x 5.5mm configuration that suits most of the popular DC power jacks in existence.
Dimensions: 3' Long, 5.5x2.1mm Barrel Connectors