Market order
A market order is an order to buy or sell immediately at the best prices available in the order book. It always fills, but you do not choose the price.
A market buy takes the lowest ask, then the next, until it is filled, so in a thin book it can fill well above the price you saw. The trader who sends it is the taker and usually pays a higher fee. Market orders are the dynamic orders that break levels.
Learn more: bid and ask explained and what slippage is.