Any transaction that has splits whose accounts are denominated in different commodities (currencies are a subset of commodities) requires an exchange rate to convert between the two. For more information please see Chapter 10 of the Tutorial and Concepts Guide.
Registers for accounts of type STOCK or FUND display four columns, Shares,
Price, Buy, and Sell (the last
two will be Credit and Debit respectively if you've
enabled Use formal accounting labels in the Accounts
tab of Preferences). Enter any two and
GnuCash will calculate the other for you; if you enter 3 and the
result doesn't balance
GnuCash will ask which one to change to
make the split balance.
Registers of other types have only Credit and Debit (the names will be different depending on the account type if you haven't enabled Use formal accounting labels) and so use the Transfer Dialog with most of the fields disabled to collect either an Exchange Rate or a To Amount (the amount in the other currency/comodity). Please see (Section 6.1.3, “Currency Transfer”) for details of using this dialog.
When committing a transaction or confirming an exchange rate the price on the transaction date in the Price Editor may be updated. See Section 8.6, “Price Editor” for details.