The gap between the buy price and the sell price, and a cost on every trade.
Every currency pair has two prices: the price you can buy at and the lower price you can sell at. The difference is the spread, and you pay it each time you open a trade.
Spreads are not fixed. They widen around news, at the daily rollover and in quiet hours. A bot tested with a tight spread may lose money when real spreads are wider, especially if it trades often.
Educational content only, not financial advice. Trading forex involves substantial risk of loss.