If you look at the meaning of semantics you will find that it is understanding the meaning of words. Lot of definitions exist and I find it is more for those interested in content writing linguistic semantics holds true.
In their efforts to understand written stuff on websites and blogs search engines use LSI or latent semantic indexing. The algorithms use natural human language processing in order to understand contents better. But one need not know all the blah blah of these terms. What is required is a sensible approach to writing contents online.
While writing on website it is a good practice to induct words that are semantically related. This way it facilitates the search engines to correctly understand the topic hence correct focusing. Another benefit is that you do not have to indulge in excessive keyword inclusion. This needs practice and understanding a good number of words in your vocabulary.
Example: Say you are writing on birds, then in your write you would certainly use the word avian as well. Hence creating a cloud of related words in your content would result in better performance. But this has to be done in a natural way as you form sentences. Just remember either you have a reasonable list of semantically related words and phrases or you limit the usage of few that you have.