Traditionally when you search for something in Google, it searches for your keyword phrase and ranks the results for relevance using a complex algorithm. It takes backlinks and other factors into consideration, and it may suggest an alternate spelling. However, the basic concept of search still revolves around searching for the phrase you type into the search box.
In semantic search the idea is to search for what you really mean by that phrase and find words and concepts that are associated with your phrase. For instance, when you search for a phrase containing "java," are you talking about coffee, an island, or a programming language?
Google has made some progress with semantic search by adding better related search phrases to results pages. They've also indicated that semantics is an area where they'll continue to invest a lot of research. However, their search results still depend heavily on keyword relevance, not semantics.