What is HDRI in Photoshop, and what does it do?

High dynamic range (HDR) images open up a world of possibilities because they can represent the entire dynamic range of the visible world. Because all the luminance values in a real-world scene are represented proportionately and stored in an HDR image, adjusting the exposure of an HDR image is like adjusting the exposure when photographing a scene in the real world.The luminance values of an HDR image are stored using a floating-point numeric representation that’s32 bits long (32‑bits-per-channel). The luminance values in an HDR image are directly related to the amount of light in a scene, so it doesn't apply to 16‑ and8‑bpc image files which  store luminance values only from black to paper white, reflecting an extremely small segment of dynamic range in the real world.