Buyers and sellers meet at different prices
At a given moment, buyers may be willing to pay up to £9.98 while sellers are willing to accept no less than £10.02. The first number is the best bid and the second is the best ask, sometimes called the offer. The 4p gap is the bid/ask spread.
A displayed market price may show the latest trade or a midpoint. That does not guarantee you can trade at exactly that number. A market order to buy normally meets the available ask; an immediate sale normally meets the bid.
Why the spread matters
Suppose you buy at £10.02 and immediately sell at £9.98 while nothing else changes. You have lost 4p per share before commission or tax. For a long-term position this may be small compared with later price movement. For frequent trading or a wide spread it can be significant.
A narrow spread often appears in heavily traded, liquid markets where many orders compete. Thinly traded shares and unsettled periods can have wider spreads because it is harder to match buyers and sellers quickly.
Orders change the trade-off
A market order prioritises execution at the best available prices, but the final price can move through several levels if the order is large. A limit order sets the worst price you will accept, but it may not execute at all. Neither order type is always better; each chooses between price control and certainty of execution.
The visible best bid and ask also show only part of the order book. Available quantities matter. During fast news, quotes can change before an order reaches the market.