$.$ I've given you a limit order book to the right. Limit buys are the bids $-$ people are willing to buy X shares at that price. Limit sell orders are the asks. People are willing to sell X shares at that price. Note that, accordingly, bids are less than asks.

So$,$ how do trades ever happen if bids are less than asks? Don't people need to agree to a price? Let's walk through some trading and see what happens.

a$.$ Suppose that a market order to buy $100$ shares comes in$.$ What happens?

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b$.$ What would the price of the next buy order be$?$ How about if the market order was for $600$ shares?

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c$.$ If you were a dealer, making a market in this stock, how would you respond to a sudden stream of buy orders coming in$?$ Remember, the dealer is putting what they call "resting" limit orders in $-$ prices they are willing to buy or sell at$.$ They can change these, if they want to$,$ just like any firm can change their prices.