It is a concept best explained by example.
When you think of a hand, you probably think of something like this.
This is certainly a recognizable image, and is perfectly good for the purpose of communication, however, it's very unlike a real hand. It's really just a symbol for one, and is missing a lot of the subtle aspects of a real hand.
We have symbols like this for every object we see, and when we try to draw things from our imaginations, we start with these symbols. However, the human brain cannot possibly remember the shapes of any given object under every possible condition, so in order to really be able to draw well, we have to learn about how these objects are actually formed.
In conclusion, the purpose of drawing from observation is to replace the symbols in your head with more tangible versions of real objects.
Don't worry if this doesn't make much sense at first, it will become more clear with practice.
At any rate, try doing some quick sketches of these, for practice.
http://www.dailydiapers.com/megapics...n&cat=0&pos=13
http://www.dailydiapers.com/megapics...n&cat=0&pos=38
http://www.dailydiapers.com/megapics...n&cat=0&pos=20
