The lowest price currently available from a seller. If you buy at market, you transact against the ask.
Full Explanation
The ask is the other half of the market's two-price reality. It is the lowest price at which someone is currently willing to sell. When you place a market buy, your order meets the best available sell liquidity, which is the ask.
Because many retail forex charts display the bid, the ask is usually slightly above the number you see on the chart. That is why a new long position can appear negative the instant it opens: you bought at the ask while the chart is showing the bid. Nothing mysterious happened and the broker did not invent an extra price after you clicked. You simply crossed the spread to demand immediate execution.
Mechanically, the ask is the lowest shelf containing resting sell limit orders. If buy aggression consumes the available orders there, the ask relocates upward to the next price where sellers are waiting.