A market order doesn't specify a price at all: it instructs the exchange to fill immediately against whatever orders are currently resting on the book, working through the available depth until the full size is filled. That's what makes it fast and (for a small order in a liquid market) usually cheap, and also what exposes it directly to slippage: a larger order, or a thinner order book, means the fill walks further up (or down) the book before completing, at a progressively worse average price.
Market orders are "taker" orders (they remove existing liquidity from the book rather than adding to it), which is why many exchanges charge a higher fee for them than for limit orders.
A signal-driven strategy generally uses market orders when the entry/exit timing itself is the edge and waiting for a limit fill risks missing the move entirely, accepting some slippage as the cost of that certainty.
Educational content, not investment advice. zengtrade is paper-first and non-custodial.