Every forex trade is really two trades — you buy one currency and sell another. Each currency has its own overnight interest rate, and holding the position past the daily rollover (usually 5pm New York time) means you either earn or pay the difference.
If you are long a currency with a higher interest rate, you receive a positive swap credit. If you are long the lower-rate currency, you pay. Swap rates are typically tripled on Wednesday nights to account for the weekend, since FX settlement is T+2.
Islamic (swap-free) accounts remove swap entirely for religious compliance, usually replaced with a flat administrative fee after the position is held past a certain number of days. Several brokers — notably Exness, XM, and HFM — offer proper Islamic accounts with no time limit or administrative fee.