A pair (or trading pair) refers to the two cryptocurrencies used to make an exchange or trade on a cryptocurrency exchange. For example, if you are selling USDT (Tether) to buy Bitcoin you are using the USDT/BTC trading pair. This means that utilizing this pair allows you to sell Tether for Bitcoin or sell Bitcoin for Tether.
A trading pair does not necessarily need to be between two cryptocurrencies. For example, it can be between a cryptocurrency and a fiat currency such as the U.S. dollar.
Why do we need pairs?
To measure and to enable conversions between currencies. Whenever you buy, sell or trade a cryptocurrency you are trading it for another currency such as dollars (fiat) or Ethereum (crypto).
Popular trading pairs:
Popular trading pairs include USDT/BTC, BTC/ETH and USDT/ETH.
Pairs are generally popular due to the amount of liquidity they have. The pairs given above are all examples of high liquidity trading pairs, meaning there is a large supply of currency available to trade between.
A pair with low liquidity generally involves a less well-known cryptocurrency. While it is definitely possible to trade a low liquidity pair it will generally take longer and more involve more slippage (change in price compared to what you offered) than a high liquidity pair would.