Join is how they describe getting started knitting in a new place after the yarn has been cut. All you need to do is know where you want to start knitting, hold the piece on the needles so that that place is on your left needle ready to start in on, take a hold of the yarn coming from your "ball" and start knitting. That joins, or attaches the yarn to get you going again. Leave a tail to work in later. You may have a loose spot there on the edge for a few rows, but pulling on the tail will tighten it up and when you work in the tail it will "heal", so don't worry about it.