It is not photoshop RGB. it is actually called adobe RGB. as we know the color are coded in digital world. and in digital world we can not reflect the limitless colors that presents in nature. Since the ISO standard color RGB code has some limitations of reflecting the natural color, Adobe as the primary digital photo editing software vendor created their own standard. It is called Adobe RGB. This color space has a wider range, which contains more color. In plain word, it makes digital photos more colorful and more accurate.
Above is my understanding though. You can google out details.