Swap fee
A swap fee — also called rollover — is the overnight financing charge, occasionally a credit, applied to a leveraged position held past the daily rollover time.
The fee exists because a leveraged position is carried on borrowed exposure (see leverage and margin). In forex the swap reflects the interest-rate gap between the two currencies in the pair: holding the higher-yielding side can in principle earn a credit, though broker markup means many retail positions pay in both directions. Index and metal CFDs charge a financing rate on the position's value. Rollover is conventionally 5pm New York time, and most brokers apply a triple swap once a week — typically Wednesday for forex — to account for weekend settlement.
Per night the number looks trivial, which is exactly why it deserves arithmetic. Day traders who are flat by rollover never pay it; for swing trading, it is a running cost that scales with holding time and position size.
Example
Suppose a 1-standard-lot position carries a swap of −$7 per night. A two-week hold spans ten trading nights, charged 14 times once each Wednesday's triple rollover is counted: 14 × $7 = $98. If the trade targets 100 pips — $1,000 on one lot — financing has quietly consumed about 10% of the target. Had the swap been a +$2 credit, the same hold would have added $28 instead.
In a prop-firm challenge
Challenge P&L is measured net of swaps, so a hold-and-wait style bleeds against the profit target a little every night, and losing positions held "until they come back" cost money while they wait. Before an attempt, check three things in the rulebook: the swap rates on your instruments, whether swap-free accounts are offered, and whether holding over the weekend is allowed at all — at some firms it is a rule violation rather than a cost.