Loonie is the forex slang term for the Canadian dollar, which uses the code CAD. The name comes from the loon bird on Canada's one-dollar coin.
Traders use Loonie most often for USD/CAD, one of the major pairs, where USD is the base currency and CAD is the quote currency.
The Loonie moves on Bank of Canada policy, Canadian inflation and employment data, oil prices, US economic conditions, rate expectations, and global risk sentiment. Canada is a major oil exporter, so the Loonie often tracks crude oil prices.
You see USD/CAD = 1.3500, so one US dollar equals 1.3500 Canadian dollars.
If USD/CAD falls to 1.3300, traders say the Loonie has gained, because the Canadian dollar has risen against the US dollar.