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.
On the other hand, the 'Target Spread' section on each instrument's information tab is what the spread amount would be for that instrument under normal market conditions.