Did you even know that Google was scanning your Gmail messages and then targeted advertising to you based on the contents? They were. Now, Google says they won’t do it anymore. Well, kind of. They’ll still scan and use the info, but just not serve ads.
Consumer Gmail content will not be used or scanned for any ads personalization after this change. This decision brings Gmail ads in line with how we personalize ads for other Google products. Ads shown are based on users’ settings. – Google Blog Post
You can opt out of targeted ads altogether on Gmail and change your preferences by going to https://myaccount.google.com/
How many people could this affect? Google says it counts more than 1.2 Billion Gmail users. The new policy is set to go into effect later this year.