A swap is basically the overnight financing charge or credit applied when a trading position stays open after the broker’s daily rollover time. I noticed that online forex brokers UK can calculate it differently depending on the currency pair, position direction, interest-rate difference, and their own trading conditions, so it is worth checking before holding trades overnight. For short-term trades it may barely matter, but for swing positions kept open for several days, swap costs can gradually affect the final result. I usually check the broker’s contract specifications in advance, especially before weekends when rollover charges may be adjusted.