Pretty much like it says on the tin. PetaPixel writes,
There isn’t a filter in the app that lets you selectively see only Portrait mode photos, but the new option in the Edit menu will be present for any Portrait shot.
Download the latest version of Google Photos for iOS to get started with this new feature. Depth editing is already available on the Pixel 3, Pixel 2, and Moto phones that have depth photo support. Google says it’ll also be adding more Android devices soon.