Swap, or rollover, is the interest paid by or to a trader for holding an open position overnight.
Swap is an unavoidable part of forex trading as every trade requires you to borrow one currency in order to buy another, and interest rates are applied; in every transaction traders pay interest on the currency that is borrowed, and get paid interest by the broker for the currency that is bought.
Let's give an example. Say you have a long AUDJPY position and keep it open overnight. If the interest rate of the AUD is 4.5% and the one of the JPY is 0.1%, you will be earning swap of 4.4% because your broker is paying you the interest on the AUD, and you are paying back the interest on JPY.
Below is a list of up-to-date swap rates for the following currency pairs: EURUSD, USDJPY, GBPUSD, AUDUSD, USDCHF, EURJPY, EURGBP, EURCHF, USDCAD, NZDUSD.