Bid / Ask
Bid = highest price a buyer will pay. Ask = lowest price a seller will accept. Gap between them is the spread.
Market buys fill at ask; market sells fill at bid. Charts usually plot bid by default, so every buy entry fills a spread's worth above the visible candle.
Learn to actually use Bid / 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.