Most foreign exchange rates are quoted against a widely traded currency, most commonly USD.
For example USD/EUR, USD/JPY.
The related cross rates are the foreign exchange rates between the other related pairs of minor currencies, calculated via the USD rates.
For example the EUR/JPY rate, as calculated via the USD from the USD/EUR and USD/JPY quotes.
Cross rate bid-offer spreads are normally wider than the bid offer spreads quoted against USD because they incorporate two bid-offer spreads against the USD, not just one.