Google makes most of its money from showing ads, both on its own services and on third-party sites running Google's ad network. When you use Google products, you have to be okay with some of the things you do being used to inform the ads you see. Today, Google is making a new tool available to users that lets you see and control the information Google uses to show you ads, but it comes with an (optional) expansion of Google's advertising personalization.
You can access the new ad settings control panel from your Google account settings. Right at the top is a toggle to enable or disable all ad personalization. Turn that off, and the Google ads you see will no longer be based on your data (when you're logged in, obviously). As Google says, this makes the ads "less useful" to you. Sure, if you think ads are useful in the first place. The new setting is right below the main toggle—an opt-in that lets Google use your account data to tailor ads on third-party sites; previously it only did this on Google sites. This doesn't mean those sites get your data, but Google is still making this an option. You'll be asked about this setting when you first access the ads settings and can choose to keep everything unchanged by clicking "More options" at the bottom of the screen.
The ads settings page also includes a list of all the ad topics in which Google thinks you might be interested. My account has a great many things I am not interested in, but you can turn off anything in the list. If you really want to, you can even add new topics manually. Why Google would even include this option is beyond me. Are people really going to enter their interests into this page for advertising purposes? Lastly, there is general demographic information, which you can alter as well.
If you're already a little uncomfortable with Google's advertising practices, you'll probably want to leave that new setting off. The ads setting page actually makes it easier to disable or modify the use of your data, so I suppose this is a win for more privacy-minded Google users.