It basically means that they already track so much information about you that they don't need to monitor the contents of your email anymore. Now Google knows:

It basically means that they already track so much information about you that they don't need to monitor the contents of your email anymore. Now Google knows: - all your searches (know your interests), - a great percentage of the pages you visit (ads and analytics) - all your contacts and how frequently you connect to them (metadata in gmail) - the places you visit (geolocation in Android) - Google logins (know the sites that interest you the most) Your email contents is completely unecessary.