items.png-atlas is a texture atlas used to store all item textures.
items.png-atlas is a fixed-width 1024×512-pixel file that is split into different 16×16-pixel squares for most blocks and items. Only the top left 1024×512 corner is actually filled with blocks and items in the base game default textures.
Like other atlases, items.png-atlas can be produced in the game's screenshot folder by pressing F3 and S in 1.19.4 Pre-release 3 and onwards.