A tonneau cover is a truck bed cover that is used to secure your cargo in the back of your pickup. Tonneau covers range from soft and flexible to hard and rigid, allowing you to pick the perfect accessory for your specific needs. Soft covers are generally lighter, cheaper, and easier to install than their 'hardtop' counterparts but they may offer less security. On the other hand, a 'hardtop' cover is more rigid and generally offers greater protection but they are heavier and more expensive.
Tonneau covers get their name from the French word "tonneau," which means barrel, cask, or container. They are designed to fit the standard shape of a pickup truck bed, and come in a variety of styles.