A spread is the difference between the buy (ask) and sell (bid) prices quoted for an asset. The spread is a key part of CFD trading, as it is how derivatives are priced.
For example, AAPL's bid price is $147.25 and the ask price is $147.30. In case you want to buy, you can buy with a price $147.30 (best offer). In case you want to sell, you can sell with a price $147.25 (best bid). $0.05 is the spread.