Ask
The lowest price a seller is willing to accept. Your buy orders fill at the ask.
Market buys fill at the current lowest ask. The ask is always higher than the bid. On a fast market, the ask can jump far above the last-visible bid in milliseconds - which is where slippage comes from.
Learn to actually use Ask.
Definitions are the easy part. The free first five modules put this on a real chart and make you do the work. No card required.