An exposure is what happens in the camera from the time the shutter opens to the time it closes.
525 exposures basically means this happened 525 times. You can combine any number of exposures and some cameras have modes to automatically do that for you. With multiple exposures you can any one of:
- Panorama Stitching
- Exposure Fusion
- HDR Blending
- Focus Stacking
- Image Stacking - This is one most often used with astrophotography. This creates a long exposure by adding up short ones. You would need to take 525 consecutive exposures and the Image Stacking software will blend them into one.
- Multi-Frame Noise-ReductionMulti-Frame Noise-Reduction - This can also be used for astrophotography to reduce the noise in long low-light exposures.
- Compositing (Overlay)
...and I probably forgot some but all of those can be used to produce image in very low-light, although an HDR / Exposure-Fusion bracket of 525 would be a bit much. Note that you can combine techniques which causes the number of images needed to multiply.