The spread is the difference between the bid and ask prices of an asset at any given time. The bid price is the maximum buyers are willing to pay for an asset at a given time. The ask is the lowest sellers are willing to take to sell their asset.

For example if sellers are willing to sell BTC at $8505, but buyers are willing to sell BTC at $8495, then the spread is $10 ($8505 - $8495).

It's essentially how much one would lose if they bought and sold an asset at the same moment.

The spread tends to be very small on assets with large market caps. For example, Microsoft (the most valuable company in the world at greater than $1 trillion) could have a spread of only a few cents, or a fraction of a percent. Whereas the spread on a random altcoin could be extremely large, reaching something like 20%. Meaning the value of the asset would have to increase by more than 20% for you to make any money.

The spread tends to increase (widen) when trading volume is low, when the market is extremely volatile, and when an asset has a low market cap. The spread tends to decrease (tighten) when trading volume is high, when the market is stable, and when an asset has a high market cap.