What is "spread" or "a spread"?

When you view the price of an instrument you will see a sell price and a buy price.
Usually, the price to the left is the 'sell price' (bid) and the price to the right is the 'buy price' (offer). The difference between these two prices is the spread so in this example the spread is 0.25.

These bid/offer spreads are also used when trading Shares or exchanging Currency.
It means that if you bought and sold one unit you would buy at the higher price and sell at the lower price, and so 'lose' the spread.
The tighter the spread, the less attrition cost you will suffer from trading in and out of positions.