In October 2011 Google introduced SSL encryption for people searching the web while signed into a Google account. Initially this only covered people using Google.com for searches but is now going to be rolled across other Google domains such as Google.co.uk.
The SSL encryption is intended to improve security and privacy but has been causing concern for webmasters as the change means they can no longer pick up keyword data via their analytics. Keyword data is extremely valuable for webmasters as it allows you to see the search queries that are driving traffic to your website.
With SSL encryption this keyword data is no longer made available and is now marked as (not provided). This change only applies to people searching with Google who are also signed into a Google account (eg Google+ or Gmail). Controversially this change does not apply to Google ads.
The change does however offer us information about how many website visitors are searching while signed into Google. The search results of people signed into a Google account can be subject to far higher levels of personalisation (where search results are adjusted based on information Google knows about that user). The ability to gather information on those visitors may go some way to compensate many webmasters for the loss of keyword data.
The amount of traffic that will be going to websites via SSL search will vary between websites but Googles Matt Cutts estimated the overall percentage to be in the single digits. In the meantime David Harry wrote a great article on Search News Central about how to make sense of Googles (not provided) data.