The boltrope (that's what it is called) often gets torn by sharp edges of the track when entering or exiting the slot-especially when you pull hard and it is pinched or at a bad angle when hoisting. As Matt says, it is simply dacron over a firm rope. Here's a video from Sailrite
https://www.youtube.com/watch?v=kGhij79W4QY showing how to do the repair. There are other you tube videos showing similar things. You can also purchase a reasonably inexpensive sewing machine on eBay that can do the job--look for "walking foot industrial". You can also purchase boltrope by the foot to replace the entire boltrope, but you really need a zipper foot on your sewing machine to do it right (and it is a 2-3 hour job since you have to take all the battens and batten pocket hardware off the luff. Finally, you could "finesse" the repair and simply add slides to use in the track instead of a boltrope.