The “Shanghai” is an index measuring all shares that are traded on the Shanghai Stock Exchange (China).
The Shanghai Index - also referred to as the SSE Composite Index or simply “the Shanghai” - is an index measuring price changes of all A and B shares traded on China’s Shanghai Exchange. Many foreign investors have only limited access to trading shares on China’s main exchange, which makes price discrepancies likely.
For that reason the index is not a great barometer of the overall Chinese economy, and also because many state run companies (which comprise a large portion of the Chinese economy) are not publicly traded.