Put simply, fiat is currency that has been issued or declared as legal tender by a government. As you can’t spend paper money on the blockchain, stablecoins have been created as their digital version.
Stablecoins are issued and backed (pegged) against real dollars at an equivalent rate of 1:1, with the real dollars held by regulated custodial US correspondent banks. Unlike Bitcoin, which is volatile, stablecoins trade in tandem with real dollars and can always be changed back to paper at a rate of 1:1.
If you don’t own stablecoins or any other digital assets, you can purchase some directly on the platform using your credit card or bank account.