I can't understand the concept of 'meta'.
When should I use it?
I don't have to use and understand it only if I make plugin?
Well, think of it this way. The item code for a wood block is 17. Notice there isn't a data value for every single wood type(birch, normal, oak etc.) The sub-block data that specifies the properties of a block is the meta data, like 17:0, 17:1, 17:2, 17:3.
It is basically like a sub-directory so to speak.
Is this concept clearer in your head now?