Like it says.
Semantics literally means using "meaningful" language; in Web Development, this basically means using tags and identifiers which describe the content.
For example, applying IDs such as #Navigation, #Header and #Content to your
The main benefits are in future maintenance; you can easily change the layout or the presentation without losing the meaning of your content. You navigation bar can move from the left to the right, or your links displayed horizontally rather than vertically, without losing the meaning.