BRL is the currency code for the Brazilian real, the official currency of Brazil. In forex, BRL represents the Brazilian real when it trades against another currency.
BRL trades mainly against majors such as the US dollar and the euro. In USD/BRL, USD is the base currency and BRL is the quote currency, so the pair shows how many Brazilian reais buy one US dollar.
BRL exchange rates move on Central Bank of Brazil policy, inflation, interest rates, commodity exports, fiscal policy, political risk, and emerging-market sentiment. As a commodity-linked emerging-market currency, BRL can swing hard on commodity prices and politics.
You see USD/BRL = 5.00, so one US dollar equals 5.00 Brazilian reais.
If USD/BRL rises to 5.20, the US dollar has gained against BRL, or BRL has weakened against the US dollar. If USD/BRL falls to 4.80, BRL has gained against the US dollar.