I am using Google Analytics, now here when I go to "Traffic Source Overview" under that it shows Keyword as "(Not provided)" which is almost 40% of my traffic source. Now more than 90% of search engine traffic is from Google and still out of that for more than 40% of keywords are "(Not provided)".

Can anyone explain me what is going wrong here or how can I get that data? Because that comes as 1st option and is biggest keyword in the list. Will that be some crawler or secure google search?