Built upon the Safe Browsing API, the new feature introduces protection against malicious downloads. If a download link appears in the Safe Browsing blacklist, Chrome and Chromium will warn users against downloading -- a save button is still presented, of course, in case you're convinced a file is perfectly safe to download.
We'd like to see something a bit more eye-catching than the red warning icon -- like perhaps painting the entire bar red. Many of the people a feature like this aims to protect probably won't notice the icon or change in wording as they'll be focused on clicking the save button.
Google is initially making download protection available to Chrome dev channel users, and you'll likely see it in Canary and Chromium snapshot builds as well. After thorough testing, beta and stable users will be next in line.