Require function

In Solidity, require is a function used to check a certain condition and revert the transaction if it evaluates to false. It can be used to verify inputs, pre-conditions, and other conditions that must hold for a function to execute successfully. If a require statement fails, all changes to the state are rolled back and the remaining gas is refunded. Here's an example:
function buy(uint256 amount) public payable {
require(msg.value == amount * price, "Incorrect payment amount.");
// ... rest of the function
In the example above, the require statement checks if the value sent in the transaction is equal to the required amount, and if not, it reverts the transaction with the specified error message.