A camera function that outputs a file with much more information about the original scene than a typical photo file (JPEG or HEIF). It's called "raw" because it outputs the original ("raw") data from the camera sensor directly to a file, without the extensive processing that's normally applied to choose the correct exposure and color balance, as well as apply HDR, etc.
Software that can read RAW files can be used to process the photo and choose the correct exposure and color balance long after the photo has been taken, then output a normal photo file (JPEG, etc.) with the desired exposure and color balance. It's almost like being able to go back in time and take the photo over again.
For example, consider a situation where you take a photo with a camera that saves both JPEG and RAW files simultaneously, (as some can do), but the camera's auto mode gets the white balance and exposure very wrong. Editing the JPEG could correct the issues only a small amount before looking "wrong", because the JPEG file only contains the brightness values and colors you can already see. The RAW file, however, contains a much wider range of color and brightness than you can see at one time, so editing that file can turn an under- or over-exposed image into a correctly-exposed one, and/or correct white balance dramatically. Different HDR algorithms can also be applied after the fact using a RAW file.
There are some standards for RAW file formats, (such as DNG, digital negative,) but many cameras (including phone cameras) have their own unique RAW formats. The software used to a edit a RAW file must support the specific camera that took the photo.