Firstly, it has nothing to do with the particular future contract you mentioned. Every underlying has a SPREAD, no matter if commoditiy, currency, stock, index etc. The spread is the difference between your bid and ask price. The spread is what you pay to the broker as sort of a fee.
You can basically assume that the difference between the bid and ask price is what the trade is going to cost you, no matter what.
Logically, ask is for buying, bid is for selling. It's the price your order will be filled at.
