Generally, the spread is the difference between the Buy (ask) and the Sell (bid) prices. Depending on the market conditions (volatility and liquidity), the spread can vary throughout the day -'Floating', or it can be set primarily for some instruments -'Fixed'.
Larger shares, currencies, and commodities can have a very tight spread as they are highly liquid markets, and instruments that don't trade much will have lower liquidity and respectively wider spread. You can check the average spread for a given instrument within a predefined period of time on our Trading Instruments' page here.