Learnability is defined as the capability of a product to enable the user to learn how to use it. In UX design learnability is a key usability feature that is often disregarded.
Learnability is often overlooked as a key feature in website design as it is quite hard to test. An accurate way to test learnability would be to have the same user use the website over a long period of time testing the interface different times. This is often avoided as it is both consuming of time and money.
Although learnability is an important feature for all websites it is not the most important feature for all websites and some apps will have a much more leaning cure than others as the website will be more complex. Different tasks will lead to different learning curves depending on the task itself. However, all websites should take into account learnability in order to be effective at what they do.