The reef knot, also known as the square knot, is a type of binding knot used to securely join two lengths of rope or string. It is easy to tie and untie, making it popular for various applications, including sailing and camping. The knot is formed by tying two overhand knots in opposite directions, creating a flat and stable structure. This knot is particularly useful for securing bundles or packages, as it holds well under tension. However, it is not suitable for joining ropes of different sizes or for critical applications, as it can slip under certain conditions.