Summarize this article in one sentence.
If you’re not already signed in to your Google account, follow the on-screen prompts to sign in now. If your photos back up to Google Photos at their original (highest) quality, they count against your total Google Drive space. This method teaches you how to switch your photo backups from full resolution to “High quality,” which still provides great quality at a smaller file size. It’s the first option on the screen. If this option was already selected, there’s no reason to change anything. It’s at the bottom of the first section. A pop-up message will display how much space you can recover by switching your photo quality.  If you uploaded any photos to Google Drive without using Google Photos, those files will not be altered. This will also compress photos uploaded to other Google products like Blogger, Picasa, and Google+. Google Photos will now reduce the quality of your photos. For most people this will not be noticeable. Once the process is complete, your reclaimed Drive space will be ready to use.

Summary:
Go to https://photos.google.com/settings in a web browser. Select High quality (free unlimited storage). Click RECOVER STORAGE. Click COMPRESS.