For example, if you are searching for something which has more than one meaning, if a search engine knows the meaning within the context of your search query, and it can match this with contextual information from its index, it can serve you more relevant search results. If you've been around search engines for a long time you'll probably remember the days when it was totally hit and miss whether you would actually get results which were relevant to your query or not.
As we're a fair way from artificial intelligence taking over our search engines and being able to make these interpretations themselves, the responsibility lays squarely at the feet of web developers to ensure that as well as encouraging well written, optimised content, that you implement semantic markup to give the content a context.
We are in the process of developing a range of tutorials and documents which will be available through our site in the coming months, but in the meantime why not check out my video from the Joomla! World Conference where I speak about the importance of semantic markup, authorship, and of course how you can implement it in your websites.