I think vim is user friendly but beginner unfriendly. For whatever task I want to do with text, vim will provide the easiest way to achieve it. However, to anyone that is not experienced with it, it will be the hardest way to achieve it.

To put another way, it probably takes a couple minutes to learn and be proficient in windows notepad, and a couple months to learn and be proficient in vim. However, for a task that may take a couple months in notepad, it might take a couple of minutes in vim.

I take beginner-friendly to be a more specific form of user-friendlyness. The usefulness of the concept is that it's common to see a UX and say it's not user-friendly, because it's not beginner-friendly. However, not all users are beginners. To someone that is proficient in both vim and notepad, vim is more user-friendly than notepad, because it is more friendly to him, the user, since it allows the completion of any task far easier.

"User Friendly" depends on the type of tasks that the user intends to accomplish, and the frequency of those tasks. The things that expert users intend to do are not necessarily the same things that beginner users intend to do.

Essentially it is about the path-of-resistance between you and your goal. A beginner friendly UI might do X, Y, and Z. But complex tasks like scheduling X, Y, and Z, automating them based on other events, or even performing them rapidly in sequence, are likely to be given a higher path of resistance.

As in, you've never heard of it and suspect they've just made it up, or you're aware of the concept but have an unorthodox opinion about it? It's a very pervasive concept among UX people. If the former, try googling "learnability vs usability" or somesuch?

The key term to search for is "discoverabilty" although there are other important factors at play