This is a term I use often and will use often here.
It is about using the tags given us in HTML in the way they were meant to be used. I discussed this in my Right tools for the Job! thread.
Molly Holtzschlag as written a great article about what semantics mean and why they are important.
From the accessibility side it simply creates a logical structure of information for those who may not be able to see or understand your graphical highlites. So sematically correct use of tags make the meaning of the information far more clear to those who may have difficulties otherwise. They may be confused why a tag is a H3 when there is not preeceding H1 or H2.
It is a good read: The meaning of Semantics by Molly Holtzschlag