"Camera Format Changed to High Efficiency your Mac is no longer syncing with iCloud photos, camera will capture in high efficiency Format”.

The format is one of the settings for the camera.

On older system versions the Camera.app has been taken the photos as JPEGs, but since iOS the Camera can use the High efficiency format. This format needs only half the storage of a JPEG of the same quality. The file extensions of these photos is .heic for photos and .MOV for videos or Live Photos.

You can change the setting back to "Most Compatible", if you prefer JPEGS and your Mac is still running OS X 10.11. You need at least macOS 10.13 High Sierra on a Mac to be able to handle the High Efficiency format.

The relevant settings are explained here: Using HEIF or HEVC media on Apple devices - Apple Support

• The Setting > Camera > Formats determines of the Camera is taking the media in High Efficiency format or in a more compatible format
• The Setting > Photos > TRANSFER TO MAC OR PC determines, I High efficiency Media will be converted to a more compatible format, when you transfer them using a USB connection.