A trading pair is two assets that can be traded against each other, usually on an exchange. They are often displayed as such: BTC/USD. When they are displayed as such, the first asset is called the base asset and the second asset is called the quote asset. In the case of this BTC/USD trading pair example, BTC would be the base asset and USD would be the quote asset. Think of it like this: when the trading pair is BTC/USD, the prices are quoted in USD.