The abbreviation HDR (sometimes seen as HDRI) stands for high dynamic range (imaging or imager) and refers to the capture or generation of digital images with a high dynamic range. Images captured using conventional means, on the contrary, are designated as LDR, or low dynamic range.
More information on the different HDR methods like kneepoint HDR, interleave HDR and Log mode can be found in our TechTip "3 ways to an HDR image".