Bid
The highest price a buyer is willing to pay. Your sell orders fill at the bid.
When you click sell at market, you fill at the current highest bid. Charts typically plot the bid price by default. The bid is always lower than the ask - the gap between them is the spread.
Learn to actually use Bid.
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.