The lowest price any seller is currently willing to accept. When you buy, you transact at the ask.
Full Explanation
The ask is the other half of the two-price reality of the market. It represents the best available sell offer — the price you pay when you buy. Because the chart typically shows the bid, buying always executes at a price above what the chart displays. The moment you enter a long position, you are already slightly underwater relative to the chart number. This is not a broker problem — it is the mechanical reality of a two-sided market.