What it means
When you buy at market, you pay the best ask, which is always a little above the bid.
In practice
The ask is where a market buy order fills and where a sell limit order waits. If you want to buy without paying the spread, you place a buy limit at or below the bid and wait for a seller to come to you.
On many platforms the chart is drawn from bid prices or last traded prices. That matters for short trades: a short is closed by buying at the ask, which is higher than the price the chart shows.
What to watch for
A stop on a short position can be triggered when the spread widens, even though the chart never printed that price. Leave room for the spread when placing stops above a level.