The spread is the gap between the bid price (what you can sell at) and the ask price (what you can buy at). If EUR/USD is quoted 1.0850 / 1.0851, the spread is one pip. You pay that one pip as a transaction cost the moment you open the trade.
Spreads vary by broker, account type, pair, and time of day. ECN and raw accounts can show spreads as low as 0.0 pips on EUR/USD during the London session, but charge a separate commission — usually $3 to $7 per standard lot per side. Standard accounts have wider spreads (often 0.8–1.5 pips on EUR/USD) and no commission.
Spreads widen during news releases, over weekends, and when liquidity is thin (late Asian session). A broker that advertises "from 0.0 pips" is quoting the best case, not the average — always check published average spread reports before choosing.