When search users are logged in to a Google Product (e.g. Google+, Adwords, Gmail, etc.) when they use Google their web search is executed over SSL. This prevents referral information (including search terms) from being passed to Google Analytics, resulting in the search term being recorded as “Not Provided”. You can use other means to find the search terms that bring traffic to your site, such as Google Webmaster Tools (now called Google Search Console) though.
Google Analytics is a free, powerful web analytics tool, but do you need it? Are there any alternatives? What implications are there for privacy?