The “spread” a broker charges is the difference between the BID and the ASK price of a currency pair at any point in time.
A floating spread and/or a fixed spread is used by various FOREX brokers for their basic means of earnings, rather than a commission.
A fixed spread remains the same in any market situation. A floating spread will fluctuate depending upon market conditions.
Some brokers will have a fixed low spread for certain currency pairs, usually these are highly traded currency pairs such as EUR/USD or USD/JPY and a floating spread for other currency pairs.