The spread is the difference between the price you can buy at (ask) and the price you can sell at (bid). It is one of the main costs of trading: every position starts slightly negative by the size of the spread. Tighter spreads mean cheaper entries, and spreads usually widen on volatile pairs like gold or around major news.
If EUR/USD is quoted 1.08500 / 1.08512, the spread is 1.2 pips — you start about $12 down on a 1-lot trade until price moves in your favour. XAUUSD spreads are typically wider.
This definition is for educational purposes only and is not financial advice. Trading involves risk.