The chromium team is currently experimenting with First-Party-Only cookies in the last chromium build². The goal of these new type of cookies is to allow web servers to mitigate the risk of cross-site request forgery (CSRF) and related information leakage attacks by asserting that a particular cookie should only be sent in a "first-party" context.
For instance, http://evil.com can include http://bank.com/admin/transfer-all-my-money in an <img> HTML tag. That request will include cookies, and if the target doesn't do a good job defending itself (via CSRF tokens, for instance), it could execute commands on your behalf (and send all your money)
If you're not sure yet what is CSRF, here's a good introduction: https://www.youtube.com/watch?v=vRBihr41JTo
After that, you might want to read the short draft specification¹ and learn more about the motivations and the limitations of these cookies.
¹ https://tools.ietf.org/html/draft-west-first-party-cookies ² https://download-chromium.appspot.com/