Learnability is a quality of products and interfaces that allows users to quickly become familiar with them and able to make good use of all their features and capabilities. Learnability is one component of usability and is often heard in the context of user interface or user experience (UX) design, as well as usability and user acceptance testing.
A very learnable interface or product is sometimes said to be intuitive because the user can immediately grasp how to interact with the system. First-time learnability refers to the degree of ease with which a user can learn a newly-encountered system without referring to documentation, such as manuals, user guides or FAQ (frequently-asked questions) lists. One element of first-time learnability is discoverability, which 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. Learnability over time, on the other hand, is the capacity of a user to gain expertise in working with a given system through repeated interaction.
Relatively simple systems with good learnability are said to have short or steep learning curves, meaning that most learning associated with the system happens very quickly, after which the rate of learning levels off or plateaus. More complex systems typically involve a longer (shallower) learning curve. (Note: Within any system that applies standards to measurement, a steep learning curve refers to something easily learned. As displayed in a graph, for example, the steepness indicates that the degree of learning obtained rises quickly. Contrary to the term's actual definition, however, most people use the term steep learning curve to indicate difficulty, similarly to the way that a steep hill is difficult to climb.)
See Tovi Grossman's tutorial about improving the learnability of software applications: