The Bid-Ask spread is simply the difference in price between the highest Bid & the Lowest Ask.
Buyers offer the Bid & are offered the ask
Sellers offer the Ask & are offered the Bid
The closer together the two price points are the more liquid a market is considered to be, and the same logic applies to the inverse; the further apart the two prices are the less liquid the market is considered to be.
The percentage differential between the prices is a metric used to gauge the severity of supply-vs-demand dynamic.