Google Photos has the ability to back up your images to the Cloud without taking up storage while saving the images in a high-quality format. If you don’t have a fast Internet connection, it will save the images in a lower quality format and when you have good Internet connection like WiFi it will replace those images with a higher quality image.
The feature mentioned was released back in March but now they are extending this feature to videos too. In the 3.6 version of Google Photos, you should be able to notice the change as now they have implemented the backup feature to videos as well. The low-quality videos can be shared with friends and family, giving them the chance to watch the video, when you get a better connection and the high-quality video is uploaded, your friends and family would just need to open up their share folder and they can watch the high-quality upload immediately.