A market order is an order type that executes immediately at the best price possible. Market orders on Delta will always be filled assuming sufficient order book depth, potentially filling at a number of different prices depending on the number of existing orders in the book at the time.
Market orders match against the best available bids/asks in the order book to ensure the best price possible for a trader. A market order can be used when speed is prioritized over the exact order execution price. When placing a market order, a trader prioritizes immediate order execution over price/quantity control.
Example: Using a market order, you can specify the quantity you wish to allocate to an order (e.g. 10% of your wallet). You cannot specify the price however, as this is based on the best available prices in the order book. The order will execute immediately.