As you're hopefully well aware, pretty much everything you do online is tracked. Information about your activity is sold to advertisers, who then serve you ads based on a profile of you built up based on your habits. It's what keeps many services, like Google's, free. Now, Google has updated its Ad Settings to provide enhanced transparency about the whole process.
Google introduced Ad Settings back in 2009 and has been iterating on it since, introducing a tool to "personalize" ads in 2016 and refining its approach again earlier this year. The new changes are pretty significant, including a big, bold toggle front and center to turn ad personalization off altogether. It's entirely possible Google is hedging its bets here in the face of the GDPR.
You're able to see a lot of individual data points Google has on you, as well as how it came by them (it's usually from a search on Google or in Google Maps, but there are other ways). Some, like age and gender, can be changed manually. Others, like specific interests Google believes you have, can be turned off altogether. You can also see all the individual identifiers you've turned off, in case you really want to see ads based on those things again. Additionally, Google's "Why this ad?" feature, which appears as an i in a circle on some ads and explains why Google thinks specific ads are relevant to you, has been expanded and will appear on more ads.
If you're curious about the specific information Google uses to show you personalized ads, you can check out the new Ads Settings now.