Midline loops, the term is self-explaining. These are loops you can make halfway the rope instead of at the end. To form a midline loop there is no need to have access to the end of the rope. This can be extremely handy when you are working with long ropes because there is no need to pull an entire 40 meters of rope through to form the loops.
Midline loops may have various purposes. For example the Alpine Butterfly knot can be used to form an extra tie in point in the middle of the rope but it can also be used as a patch for a damaged section of the line. Midline loops can also be used when you need an extra handle in the line for someone to grab on to or when you have multiple people pulling on the line. As it is much easier to grab a loop then the thinner rope itself.