The bid is the best standing offer to buy, the ask is the best standing offer to sell, and the spread between them is the compensation market makers earn for standing ready on both sides.
It is a real cost, paid the instant you cross it. Buy at the ask and sell at the bid and you are down by the spread before the price has moved at all. On a mega cap like Apple that is a cent on a share; on a thinly traded small cap it can be one or two per cent.
The spread is also a live read on stress. It widens when volatility spikes, when news is pending and in the first and last minutes of the session, which is why market orders at the open are more expensive than they look.