A RAW file is an unprocessed digital image. There’s no single RAW format—each camera manufacturer has their format. RAW files contain extra camera sensor information, which allows advanced editing. RAW images are often uncompressed, resulting in file sizes many times larger than JPEG or HEIC images.
According to Apple:
“Apple ProRAW combines the information of a standard RAW format along with iPhone image processing, which gives you more flexibility when editing the exposure, color, and white balance in your photo. ProRAW files are 10 to 12 times larger than HEIC or JPEG files, and ProRAW files are even larger if you select 48 MP as your resolution.” Source: https://support.apple.com/en-us/
Apple has supported capturing images in RAW format since iOS 10. ProRAW is Apple’s newest format.
Apple has a great introduction to RAW here: What the Heck Is a RAW Image?