Buck is the colloquial term for the US dollar. In forex, the buck refers to USD or to the dollar's movement against another currency.
Traders use buck when discussing USD pairs such as EUR/USD, GBP/USD, USD/JPY, and USD/CAD. Saying the buck is stronger means the US dollar is gaining against other currencies.
Buck is informal market language, not an official code. The dollar's ISO code is USD, and that is what appears in quotes, platforms, and market data.
A trader says the buck is stronger today, meaning the US dollar is rising against other currencies.
If EUR/USD falls from 1.0850 to 1.0750, the buck has gained against the euro, because fewer US dollars are needed to buy one euro.