A private key is a long sequence of 64 numbers and letters, or a sequence of 12 or 24 foreign words - depending on which wallet type you have.

It is a "secret number" that allows you to use the coins inside of your wallet. 

Every Bitcoin wallet contains one or more private keys, which are saved in the wallet file. The private keys are mathematically related to all Bitcoin addresses generated for the wallet.

For example, if the public address (wallet address) is a bank account number, then the private key is the password to sign in to your bank account, which allows you to perform different actions.

It is important you back up your private key and never expose or share it with anyone. It is yours and only yours.