It means if you received an email saying "thanks for your purchase of a razor" you will get ads for razor blades. They'll scan all your emails and build an "advertising profile"; similar to that Amazon thing that happened a while ago. If they know enough about you, the ads will be extremely targeted; like an automated spear phishing mounted turret that targets your most private weaknesses. Track your period in an app and they'll sell you menstrual products on a schedule. Get a food delivery order and now they know what food you like. Email your family about an upcoming vacation and they'll sell you plane tickets and travel insurance