You need to relate that to your personal life. If you've ever decorated / designed / built something (like your house, shelf, office, school parties / functions, etc.) then it should be a little simpler for you.
Example (and I'm really imagining it as it goes) See, at first, you've got this big empty room. Now the first thing you want to do is to paint it, it could be different color or just a common base color like light gray.
You probably want to make some smaller partitions in the room right now. Such partitions may have different styles and histories. Now you want to bring in some things like posters, TV sets, Couch, Bed, Pet, etc, and they're all going to be according to your specifications (but sometime you've got to use the stuff that someone else gives you).
Now you need doors to connect rooms (some can be password protected) and windows, and so on.
This is what Web design is, and this is what CSS / CSS3 helps you to do.