The bid / ask spread is the difference between the buy and sell price.
When you buy or go long you will enter at the Ask price.
When you sell or go short you will enter at the Bid price.
The Ask price is also known as the Offer price.
Example: US30 index is quoted at 38500 / 38502. You can buy at 38502, the Ask price, or sell at 38500, the Bid price.
The difference of 2 points is the spread.