Long vs short swap
FX positions involve borrowing one currency to buy another. If the currency you buy has a higher interest rate, you earn swap (positive). If it has a lower rate, you pay swap (negative). Most retail brokers charge swap in both directions for most pairs.
Impact on EA performance
EAs that hold trades for multiple days accumulate swap costs or credits. A swing EA holding EUR/USD positions for 3-7 days may pay $1-3 per lot per night in swap. This does not appear in backtests unless the backtesting platform includes accurate historical swap rates.
Swap-free accounts
Some brokers offer Islamic (swap-free) accounts. These replace swap with an administration fee. The net cost is often higher than standard swap — verify the actual fee before assuming swap-free accounts are cheaper.