A closed cable is a different type of cable that starts and ends within the knitting - rather than beginning at one end and traveling all of the way to the other end.

An important thing to note is that before this cable motif begins, you need to use increases to add more stitches for using in the cable, and after the cable motif ends, you need to use decreases to get back down to the original number of stitches.

If you don't do this, the area where the cable motif is placed will draw in the knitting from around it and create distorted fabric. This is due to cables having a high stitch gauge (i.e. many stitches per inch), compared with the surrounding fabric - which in this case is stockinette stitch.

Knitted fabric with a higher stitch gauge is smaller in area, hence why it pulls in the sides of your knitted fabric to make it pucker and become misshapen. 

Closed cables are great for adding single motifs in your knitting, to add an area of interest. So you could put a cabled Celtic motif on the back of a cardigan, or an individual cabled heart on a sleeve, to give just a couple of ideas.

If the motif is made of up strands that have no 'ends', but instead just form a loop of some sort, then these can also be called infinite cables.

Below you will find a video tutorial that demonstrates a closed cable being knitted. This cable is a 3-strand braid within a block of stockinette stitch: