A bone folder is a dull edged hand tool used to sharpen a fold line. It is mainly used for bookbinding but was quickly adopted to origami, where sharp creases are needed.
Holding the bone folder with your hand, it allows you to apply more strength then using your fingernails, and even more, if you use your fingertips.
While using a bone folder, do not make the crease with it. First force the paper gently by hand, flatten it with a fingertip, and only if you are satisfied with the fold sharpen it with your bone folder.
Originally, bone folders were made from an animal leg bone (far left), but today you can find metal or wood ones.
Although creases get really sharp with a bone folder, it is far from being a must for folding airplanes. Usually a bone folder's greatest need is when you are trying to flatten many layers together. Generally on most templates you can crease a fold perfectly with your finger nails!