Market Impact

Market impact is the footprint your order leaves on the price. Drop a pebble in a pond and nothing happens. Drop a boulder and the water sloshes everywhere. A small retail trade is the pebble. A bank buying a billion euros is the boulder: as it eats through the sellers waiting at each price, the price climbs, and the last chunk fills worse than the first.

Big players hate it so much that they slice orders up with tools like TWAP and VWAP algorithms, or trade in quieter venues. For most retail traders it is tiny, but in thin markets it can show up as slippage.