Easy to learn
Easy to learn means that if you sit somebody with no experience down with a tool, it will be (relatively) easy for them to learn how to use it. Maybe they have instruction from somebody experienced with it, in which case it's a lot easier to learn a larger amount. But maybe they don't. Either way, it should be fairly bloodless to learn.
One useful tool for learnability is 'intuitiveness'. If something is intuitive, it's far easier to learn than if it's not. That's not to say that unintuitive is impossible, of course. Compare steering a car (with a wheel that angles the front wheels) with steering a boat (with a handle that angles a rudder). They're exactly opposite in effect; in one case, nudging the control to the right sends the vehicle to the right, in the other, it sends it to the left. They can't both be intuitive, after all. Most people would probably say the boat is unintuitive. But nobody really has any trouble learning it after a few missteps. So, intuitiveness is a nice plus, but it's not a requirement. It's doubly nebulous, since it's very subjective; different people find different things to be intuitive.
Obviousness is another nice tool, though it's also rather subjective. The "Play" button on a VCR is 'obvious', as are "Power" buttons on all most electrical devices. The use of a volume knob, or a light switch, is fairly obvious. The "Back" button in your web browser is semi-obvious; you need a basic understanding of web navigation to 'get' it, but once you have that, it's obvious.
This is difficult to design for, though. For one thing, its very dependant on the individual person's thought patterns. For another, it often depends on some background knowledge; once you have that knowledge, it's blindingly obvious and trivial, but without it, you just get confused. That makes it very difficult for the designers (who undoubtably 'get' it) to see that it's not actually obvious. Doing it at all, to say nothing of well, takes an enormous amount of work.
This is great. The only downside is it requires an instructor who knows the material well enough to teach it. If reasonable, this is the very best way to learn. Unfortunately, this doesn't happen near often enough in the situations we're interested in, since quality unpaid instruction is tough to come by in any field. This can almost be a detriment, since most people will turn to their friends for help automatically. And while you're very likely to find a friend who considers themselves an expert, the chances that they actually know half of what they claim to are absymal. Most likely, they know a lot of hot air. And the reason they know that is because they were taught by one of their friends who was full of hot air. Who learned it from a friend who was full of hot air. And so on. All in all, a recipe for disaster.
Whoo boy. Well-done documentation is a godsend. Badly-done documentation is one of the torments described by Dante. There are two important factors to bear in mind concerning documentation.
Documentation is indispensible
Anything that's not blindingly obvious to a 4-year old needs documentation. And if it is that obvious, it should probably be documented anyway, because sooner or later, a 3-year old is going to want to use it. It needs good documentation that's kept up-to-date and correct, and covers a reasonable range of the functionality. I'm not going to define what a reasonable range is; that's your job as the designer.
Documentation will never be read
Sigh. Damned users. They won't read the documentation until they're in trouble, and even then, they're likely to just give up. Well, there's not really any good cure for this. Let me just make 2 points, and move on to something less depressing.
- Keep it short. It's that much more likely to be read.
- Don't get caught within the bounds of traditional documentation. The man who invented 'Tool Tips' that appear after your mouse cursor hovers for a few seconds should be given a medal. That's a GREAT application of non-traditional documentation. Just remember to keep it unobtrusive, and neat.
What this means
Well, it means that no matter what, you're screwed :). Making something easy to learn is really tough. The smaller a subset of people you target, the easier it becomes. Unfortunately, targetting a select group becomes progressively harder when you start making stuff for everybody. The best thing you can do is play with stuff yourself. Not work with it. Play with it. Fiddle around, see what happens when you do unexpected things. Ask other people questions, and listen to the answers. And, really, it comes down to luck.
Never forget, though
... what your goal is. You're not trying to teach a product, you're trying to make a product. Don't focus exclusively on learnability. At least as important (and likely more so) is usability.