Collateral is the security that is used to secure a loan. In the case of the TRON network, collateral is provided in the form of TRX tokens. When a user provides collateral, they are essentially creating a loan agreement between themselves and the lending platform. This agreement states that the user must pay back the loan, plus interest, in a certain amount of time. If the user fails to repay the loan, the collateral will be used to cover the debt. This helps to protect the lender from any potential losses due to defaulting on the loan. Additionally, collateral can also be used to increase the lending limit for a user, allowing them to borrow more than they otherwise would be able to.