If something is fungible, you can exchange it for something else. Why is it that you can trade a twenty dollar bill for a ten and two fives? Because money's fungible!
The “g” in fungible is pronounced like a “j” or like the “g” in “fudge,” though fungible things aren't always tasty. Fungible comes from a Latin phrase meaning “to serve in place of” and that's what something that's fungible does: serves in place of what it's exchanged for.