While limited to 16 Megapixel or smaller photos, and 1080p or smaller videos, it allows you to upload as many photos or videos as you can that meet the requirements.
Google did not explicitly mention if the feature would be retroactive or only available for new uploads to Google Photos.
If you have uploaded lots of photos or videos to Google Drive, the company's cloud storage solution, then it may be beneficial to you if those files would fall under the same unlimited storage policy.
This however is not the case. A tweet to a customer inquiry by the official Google Photos Twitter account reveals that the unlimited storage option applies only to new uploads but not existing photos or videos stored on Google Drive.
This means that photos and videos uploaded to Google Drive still count against the account's storage quota.
Since there is no "export to Google Photos" option available right now, the only option you have to reduce the storage used on Google Drive is to re-upload the photos and videos to Google Photos.
This is not comfortable considering that you may need to upload (and maybe even download) Gigabytes of data again to Google servers.
While you can add a Google Photos folder to Google Drive, it appears impossible right now to move photos and images to the service on Google Drive.
It would actually be comfortable to do so. You would search for media using the type:image search option on Google Drive, mark all images found, right-click and select the move context menu entry to move them to the Google Photos folder.
Since this is not working currently, you are left with the re-upload option right now. The best option for that is to use the Google Photos Uploader that Google released some time ago.
Ideally, you have access to all photos and videos that you have uploaded to Google Drive on your desktop system. If that is not the case, you need to download the files first to it before you re-upload them to Google Photos.
The operation may take a while to complete considering that you may have uploaded Gigabytes of media files to Google Drive in the past.
This is a one-time operation on the other hand as new uploads make use of the new system automatically.
Please note that you need to delete the photos and video files that you have uploaded to Google Photos on Google Drive after the import to Google Photos completes. If you don't do that, you end up with duplicates on Google Drive that still count against the account's storage quota.