The market spread is the difference between the highest price bid and the lowest price asked for on the order book. To make it clearer, the gap is essentially the difference between the price at which people are willing to sell an asset for and the price that other people are willing to buy an asset.
Imagine negotiating a deal to buy Bitcoin from Person X. You might tell Person X that you are willing to buy one Bitcoin for $20,000. However, Person X might respond saying they feel it’s worth more than $20,000, and they are only willing to sell it for $30,000.
The difference between your bid to buy Bitcoin for $20,000 and the other person’s ask to sell Bitcoin for $30,000 is the spread.
The above was just one example, but imagine this being done on an entire cryptocurrency exchange!
The order book is essentially thousands of people mentioning the price that they are willing to buy or sell an asset for. The people with the lowest asking price and the highest bid price are on either side of the market spread.