what does 'from concentrate' mean (juice)?!
what does that mean...and what is the difference in quality (nutrition/taste, etc)
is one generally cheaper than the other
Most of a juice is water. Water is a heavy, bulky substance that is expensive to ship. So most juices are concentrated by evaporating most of the water and leaving behind a thick liquid. Then the concentrate is shipped to wherever it needs to go, and a local bottling plant adds the right amount of water to the concentrate to produce juice again.
Juices not from concentrate are more expensive (all that water was shipped the entire distance), but they're usually said to taste a little better and the concentrating process risks losing very fragile components.
A concentrate is a form of substance which has had the majority of its base component (in the case of a liquid: the solvent) removed. Typically this will be the removal of water from a solution or suspension such as the removal of water from fruit juice. One benefit of producing a concentrate is that of a reduction in weight and volume for transportation as the concentrate can be re-constituted at the time of usage by the addition of the solvent.
Using apple juice as an ingredient, think of it this way.
Concentrated apple juice: a small, heavy concentration of the juice from the apple, giving a stronger taste.
Not from concentrate: Not as much juice and a lot of water.
Think of it like shampoo. If you have concentrated shampoo, you only need a little bit of it. With unconcentrated, you need much more to do the same job that the concentrated would.