Swaps, or rollover fees, are overnight interest charges (or credits) for holding positions past the daily close ( 5:00 PM EST). They're based on the interest rate differential between the pair's currencies and your position direction (long/short) - positive if you're earning interest, negative if paying.
Some pairs have negatives on both sides. Swaps apply per standard lot only to overnight holds, not day trades. Triple swaps hit on Wednesday (most Forex) or Friday (depending on the instrument) to cover weekends.
You can view the exact daily swap rates for each pair directly in your trading platform under Symbol Specifications.