Spread = Ask − Bid. For example, EUR/USD quoted at 1.0870 / 1.0871 has a 1-pip spread.
Spread is your most direct cost. ECN brokers offer tighter spreads (0.0-0.5 pip) but charge commission; Market Makers offer wider spreads but usually zero commission.
Spreads are not fixed — they widen sharply around major news (e.g. NFP).