A jam knot is a type of knot that slips easily onto whatever you're tying, but it won't come undone until you decide to untie it. This is thanks to a backup knot that keeps the lashing from slipping, and it makes construction with logs a lot easier than endless hasty lashings that use up all your rope.
Check out how to perform this simple knot on the video below.
This is a great trick because not only does it help you save on rope by not requiring endless lashings over the original knot to keep it bound, it also makes construction when you're on your own much easier because you don't need an extra set of hands to help you keep the rope tight.
As the video says, the knot will only come undone when you decide to retrieve your rope and slide the knot in the other direction.
This is a great trick not only for survival, but for constructing little projects around your campsite, like a dish rack or a tripod for the fire.