The difference between the buy (ask) and sell (bid) price; it is the basic cost of trading.
If EUR/USD is quoted 1.10000 / 1.10012, the spread is 1.2 pips. You pay that cost when opening the trade, so you start slightly negative.
Spreads vary with liquidity: they are usually tighter during the London and New York sessions and wider overnight, on weekends and around major news.