The fallback function

This is a special type of function that doesn't have a name. It is executed whenever someone uses the .send() or .transfer() function to this contract address, similar to a default function. It is often used in ICOs to receive Ether and return the specified number of tokens for the Ether received. This allows anybody to buy tokens without having to understand and deploy the contract instance, with just the address of the contract.

Here's how it looks:

function () external payable {}

Fallback functions must be marked external to help people understand that it shouldn't be executed inside this contract by mistake. If you don't add the payable modifier, it will reject all the transactions sending it Ether.

I recommend that you should write and try different functions using the remix.ethereum.org IDE, which will show you errors and notifications about things that must be verified. That way, you'll be able to write your own contract securely.