Google intends to coordinate all their sources of information about their users and create personalized ads for them, but they are asking for your permission first. If you agree to the change, Google will combine information they gather about you from Search, Youtube and Chrome and build a profile which will outline your interests, and provide advertisers valuable information about what kind of ads and products you might be interested in seeing.
Before this time, Google has kept that type of information separate, but with this new initiative, they hope to increase the value of advertising sent to their customers. They have a new page, called My Activity, which will allow users to have some control over their profile.
Here is a screenshot showing Google’s explanation of this new program:
This is similar to what Facebook has done, but at least Google is making this option voluntary to join. Facebook forced their users to opt-out if they did not want to participate.
You can read more on the story at Business Insider.