Rabo's given you a good description of the term.
I don't personally believe it's a good idea. I think it's a waste of valuable time you could be putting into their education, and also it's a bit of a waste to put in the time & effort of 'starting' them if you're just going to turn them out to forget it for a year or more. Sure, they remember some, so there's a bit less effort to 'restart' them the following year, but not very productive.
I'm a firm believer of doing anything & everything with a horse as young as possible, obviously excluding things the horse is physically too immature for, such as lunging, weight bearing, etc. I believe it's important to teach them in a non-confrontational way & build a great relationship and attitude towards playing the 'games' you want them to learn. Apart from some *very* light stuff like short walks, I don't 'back' a horse until they're around 4-6(depending on breed & development) but by the time they're that age, they have done just about everything I'll ever require of them, and done it to a point where they're not phased by much.
The other thing is that domestic horses of any age generally get far less exercise than is good for them. Allowing them to do nothing but wander around a soft pasture or such is not great for them & within reason, the more exercise you can give them, the better.
