Raffia is the fiber produced from the membrane on the underside of each individual frond leaf of the Raffia Palm.
What I like about this particular product over some of the others is that it is wide and long. I have seen up to 56″ long and 0.75″ wide. This is a natural product so there are variations.
For bonsai use, the strands are separated into groups of 6-7 or more strands and tied at one end. It is then soaked in water for 20-30 minutes (some people go as much as an hour) and then carefully wound tightly around the length of the branch. When wet it is applied to our bonsai, it can be placed in single flat layers. As the raffia dries, it will also shrink and tighten slightly, giving the branch more support.
When a wire is wound around the branch and bent to the desired shape this product will minimize the separation that can occur between the heartwood, cambium layer, and bark.
You might be interested in reading an article about the bonsai branch design. Proper Bonsai Branch Structure