The spread is the price gap between the lowest sell order and the highest buy order in an order book. It adjusts constantly as orders are getting bought and sold while new orders are added to the book, so it varies depending on the point in time and the trading pair in question. Generally speaking, the more volume and liquidity an order book provides, the smaller the expected spread would be. Consequently, trading pairs with a subpar volume and liquidity usually exhibit a larger spread.
The spread creates an incentive to make use of limit orders for buyers and sellers alike. Limit Orders allow to buy below the lowest ask and sell above the highest bid. In case of market orders however, traders realize their buys and sells on the opposite side of the spread and therefore buy above and sell below market price.