You use the private key to send funds from an account. It is the secret half of your address and is a string of 64 hexadecimal characters. If you write down your private key differently today than yesterday, you will be accessing a different wallet. Never write your private key by hand.
A private key typically looks like this: afdfd9c3d2095ef696594f6cedcae59e72dcd697e2a7521b1578140422a4f890.
Without your private key, you cannot access your funds. To keep it secure and easier to retrieve and remember, you can save the private key using a keystore (file or string) or mnemonic phrase.