It means the final image was assembled from 525 individual photos, each made when the shutter opened and closed once. This is common in digital photography because many short exposures can be easier to manage than one very long exposure.

Possible uses include:

• image stacking: combining many short frames to simulate a longer exposure
• multi-frame noise reduction: averaging frames to reduce noise
• HDR blending or exposure fusion: combining different brightness exposures
• focus stacking: merging frames focused at different distances
• panorama stitching: combining overlapping views
• compositing/overlay: manually blending selected elements

A very common case is astrophotography: you shoot many consecutive exposures of the same scene, then use stacking software to align and merge them. This can reveal faint detail, improve signal-to-noise, and avoid some problems of extremely long single exposures.

To replicate it, choose the technique first, then capture a consistent set of frames for that purpose and combine them in suitable software. For example, for stacking, shoot many aligned images of the same subject and blend them with stacking software; for focus stacking, keep the camera fixed and change focus between shots; for HDR, bracket exposure and merge the set.