A flash loan is a cryptoeconomic primitive usually used by arbitrageurs in decentralized finance (DeFi) applications. In conventional loans, lenders require up front collateral before giving money to a borrower. Flash loans are different in that regard - Flash loans allow borrowers to take out as much liquidity as desired out of a DeFi service, without requiring any collateral at all. The borrower has to return however the borrowed liquidity (as well as a slight interest on top of it) at the end of the same transaction back. The Xfai CPMM model allows for flash loan requests from every pool within the DEX. Since Xfai has token pools instead of token pairs, the liquidity that arbitragers can borrow can be significantly higher than in other token-pair-based DEXs.
To see how to implement a flash loan, read the Implement a Flash Loan subsection of the developers documentation.