Market orders and slippage

A market order says: fill me now, at whatever prices are there. It is the fastest order and the least controlled one.

The book holds only so much at each price. If you want more than the best ask has on offer, your order takes all of it, then moves up to the next price, and the next. Each step costs a little more. You pay the average of what you actually took, not the first price you saw.

The difference between the price you saw and the average you paid is slippage. For a small order on BTC it is close to zero. It grows with the size of the order and shrinks when the book is deep. That is why a big order in a thin market is expensive.

The sounding next to the ticket draws exactly this: how far your order sinks into the book and what that costs you. It works on a recent snapshot of the real book. On a live market the book keeps moving while you click, so real slippage can differ.