iMacs of this year range have glass that is fused to the LCD panel. This is the complete display assembly with LCD panel and integrated glass cover. If the glass breaks, you need this. This is the "Retina" 4K display with a whopping 4096x2304 resolution.
661-02990 LCD for iMac 21.5-inch Late 2015 A1418 MK452LL/A, BTO/CTO (LM215UH1 SD A1) This is a replacement LCD for the Late 2015 iMac 21.5" Retina models.