Folding bones (or bone folders) are great when you have a lot of paper to fold and your fingers are sore. These tools help fold, crease, and score paper.
Today, commercially available ones are made of plastic, Teflon, bamboo, or different types of wood.
There are many types and styles of bone folders on the market. Some are made of plastic while others are made of bamboo. Some are straight while others are curvaceous. Some have a sharp, crisp edge while others have a soft edge.
Some artists prefer their origami models to have soft creases so they do not use bone folders at all. However, if you are folding a lot of origami then a bone folder will probably be a useful tool to have. This is especially true if you are making modular origami and simple folds are made repeatedly. You need not buy a bone folder, other household items can do the job just as well.
Milk-jug folding bones pieces of plastic which come from the handle of a one-gallon jug of milk. They can be found scattered on the milk shelves at some grocery stores. For the grocery store, they are garbage - but for an origami enthusiasts... they are indispensable "tools" - and they're free!
People who sew will be aware of another similar tool called a point turner which is used to turn sewing projects (collars, pockets, belts, etc) right-side-out. It can also be used to crease seams, or in this case, crease paper.