User-friendliness is the noun for the quality of being user-friendly — how easy a thing is for an ordinary person to use, understand and operate without training or a thick manual. You use it when judging designs: "the user-friendliness of the new app", "they improved user-friendliness by making the buttons bigger". It is mostly said about machines, software, websites and apps, but it stretches to anything with instructions or controls — a ticket machine, a rice cooker, a form you must fill in. It is uncountable: no plural, no "a". It has a slightly official, report-and-review feel; a friend chatting would just say "it's easy to use". Note the meaning is about the person using it, not about how the thing looks. Something can be beautiful and still have terrible user-friendliness. In the workplace people often use the word "usability" for the same idea.