A swap (or rollover) is the interest charged or credited for holding a leveraged forex position open overnight, reflecting the interest-rate differential between the two currencies in the pair. If you hold a currency with a higher interest rate against one with a lower rate, you may earn a positive swap; the reverse typically means you pay a negative swap.
Many brokers charge triple the normal swap on Wednesdays to account for weekend settlement, and some offer "swap-free" Islamic accounts that comply with Sharia's prohibition on interest.
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Add up the spread, commission and overnight swap on one forex trade, and see how many pips it must move before you break even.