Complex sentence: The phrase in the bight (or on a bight) means a U-shaped section is itself being used to make a knot, and more specifically that the knot can be formed without access to the ends of the rope.
Split into simpler sentences: The phrase in the bight (or on a bight) means a U-shaped section of line is itself being used to make a knot. Specifically this means that the knot can be formed without access to the ends of the rope.