Currency pairs measure the value of one currency against another. They are split into a base currency and a secondary quote currency. An example is perhaps the most popularly traded currency pair in the world, the EUR/USD, where its price represents how much of the quote currency, the US dollar, is required to buy one unit of the base currency, the Euro.
Each currency pair has a bid and ask price. The bid price is defined as the maximum price a buyer is willing to pay for the currency, the ask price is defined as the minimum price a seller would be willing to accept the currency for, and the bid-ask spread represents the difference in pricing between the two currencies. In some cases, forex traders prefer to trade on currency pairs with a lower or tighter spread as it reduces the overall costs involved in the trade. However, some traders prefer to trade on volatile currency pairs with wider spreads and lower liquidity in an attempt to profit from the fluctuating gaps in price.