Bonded nylon (or polyester) threads are a very strong, synthetic thread. Whereas many threads are composed of material strands twisted together, bonded nylon is also physically bonded together. This makes it a much stronger thread. The properties of it being nylon (essentially, plastic), make it very sturdy, water resistant, wear resistant, and last a very long time.
For many leather working projects, bonded thread is a staple thread used. They are available in a range of colors and thicknesses, so one can be selected to best suit the project you are working on. Nylon threads are also great for machine sewing and hand sewing as well.