上 (shàng): This character typically means “up” or “on.” Here, it's an abbreviation for 上海 (Shànghǎi), the city of Shanghai.
证 (zhèng): This character is short for 证券 (zhèngquàn), which means “securities” (like stocks and bonds).
指 (zhǐ): Meaning “finger” or “to point.” In this context, it's the first part of the word “index.”
数 (shù): Meaning “number” or “count.” It's the second part of the word for “index.”
The characters combine logically: 上证 (Shàngzhèng) is the common abbreviation for the Shanghai Stock Exchange (上海证券交易所). 指数 (zhǐshù) is the standard Chinese word for a statistical “index.” Put them together, and you get “Shanghai Securities Index,” or the Shanghai Composite Index.