The main references are the FTSE 100 in London, the DAX in Frankfurt, the Euro Stoxx 50 across the euro area, the Nikkei 225 and TOPIX in Tokyo, the Hang Seng in Hong Kong and the CSI 300 in mainland China.
Each reflects its own economy less than its own composition. The FTSE 100 is dominated by energy, mining and banks earning largely in dollars, so it frequently rises when sterling falls, and the Nikkei is price-weighted, which gives a handful of high-priced stocks outsized influence.
For a US-based reader they are mainly a leading indicator. Asia and Europe trade through the American overnight, so their direction by the time US futures reopen is the first read on how global risk appetite has shifted.