Flash loans are a type of loan that allows users to borrow funds from a liquidity pool within a single transaction, without any collateral or credit checks, as long as the borrowed amount is returned to the pool before the end of the transaction.

Lets understand Flashloans with a help of an example:

1. A user wants to borrow a large amount of cryptocurrency (e.g., 1000 ETH) for a short period of time (e.g., a few seconds).
2. The user executes a smart contract transaction that requests a flash loan of 1000 ETH from a lending platform that supports flash loans. The lending platform is a decentralized liquidity pool that is funded by multiple users who deposit their cryptocurrency into the pool.
3. The lending platform checks if the user can repay the loan by the end of the transaction. If the user cannot repay the loan, the transaction is reverted, and the user does not receive the loan.
4. Assuming the user has enough funds to repay the loan, the 1000 ETH is transferred to the user's wallet. The user can now use the borrowed funds for any purpose, such as trading, arbitrage, or paying off debt.
5. Before the end of the transaction (typically within seconds), the user must repay the 1000 ETH plus any interest or fees charged by the lending platform. If the user fails to repay the loan, the transaction is reverted, and the user loses any funds used to initiate the transaction.
6. Assuming the user repays the loan within the same transaction, the borrowed funds are returned to the lending platform, and the transaction is completed.

The advantage of flash loans is that they allow users to access a large amount of liquidity without requiring collateral or credit checks. This makes it possible for users to engage in arbitrage opportunities, liquidate positions, or take advantage of market inefficiencies that would not be possible with traditional loans. However, flash loans also carry significant risks, such as price volatility, smart contract vulnerabilities, and transaction failures.

Reference

• https://docs.aave.com/developers/guides/flash-loans