Discoverability, in the context of product and interface design, is the degree of ease with which the user can find all the elements and features of a new system when they first encounter it. That ability is an important consideration in user interface and user experience design for hardware devices, software applications and websites.
Discoverability is one component of learnability, a measure of how easily someone can find, access and make use of the components and features of a new system. Learnability, in turn, is an element of usability, which is an assessment of a product's potential to accomplish the goals of the user.
In other contexts, discoverability means a number of different things. In mobile communications, for example, it refers to the capacity of a device to be detected by another device; for regulatory compliance, discoverability requires that data be stored in such a manner that it can be accessed if necessary.
See Don Norman's TED talk, Three ways good design makes you happy: