please suggest about semantic keywords and what are benefits we can get by using this semantic keywords.
Semantic keywords are those which are related to your primary keyword. So if you are writing a page all about fitted kitchens, your article should contain words like: oven, sink, microwave, cupboards, worktop etc etc - in other words, things to do with fitted kitchens. That way the search engine understands what your page is about. People stress too much over trying to squeeze related words into their articles. Just write naturally, and those words will end up in there by themselves.