DISCLAIMER: This is not a stock picking recommendation, the tickers shown are used as example and not provided as financial or investment advice.
Usage is simple, for example to get the price of a stock, first search for it on Google Finance, then look at the quote URL it generates and copy the exchange:ticker value into the function.
In this example we try to get the quote for A200 ETF traded on the Australian ASX exchange. The URL shows ASX:A200 after the quote part, therefore we should use:
=GOOGLEFINANCE("ASX:A200")
So far, so good. Sometimes however Google does not show data for a particular ticker, or it shows data only in another currency or from another exchange.
For example, another ETF XESC shows quotes from many exchanges but is also traded on the German XETRA (XESC.DE), which is not sourced by Google. Or the CHSPI ETF traded on the Swiss Six exchange (CHSPI.SW) is simply not found at all.