What is Semantic Code/HTML you ask?
In the Design and content guidelines sub-section of Google’s Webmaster Guidelines there is a paragraph that reads “…. write pages that clearly and accurately describe your content”. The only way I know of achieving this objective easily is to use semantic markup.
Semantic markup means using html elements that are appropriate to ‘content meaning’ rather than ‘content presentation’. A simple example might be using <em> for emphasis in some cases rather than <i> for italic because <i> only tells the browser what to do and does not explain what the content represents.
Apart from helping Google there are other advantages in separating content from presentation and using semantic markup. For example much easier code maintenance and correct interpretation by other user agents like audio screen reader software.