Foldover elastic has a line scored lengthwise down the middle, so it folds over neatly and easily. Unlike most of the foldover elastic in the shop, which is meant for inner wear, the type in this listing is usually used for finishing the sleeve, leg and waist hems of jackets, fleecey pullovers, sweatpants, etc. For reference, here's a tutorial for using it, an example sewing pattern which calls for it, and an example ready-to-wear garment. Because of the fold line down the middle, it is not suitable for inserting into waistbands like regular elastic. To preserve the life of anything with elastic, avoid washing at high temperatures or tumble drying.
Cobalt herringbone is a dusty cobalt blue with a herringbone design woven in. Dark navy is, well, dark navy in colour, and it feels more like regular elastic than the cobalt, which has a bit of softer touch. The neon-edged foldover elastic listed separately could also be used for hem finishings.
