A private key is a long sequence of numbers and English letters or in its more conventional form - a sequence of 12 or 24 foreign words.
The private key allows access and actions at the public address (in the wallet). If the public address is a bank account number, then the private key is the password to sign in to your bank account and take action.
The private key is important to back up externally and never to expose to anyone. Proceed to the article about how to secure your wallet to make sure your wallet is properly secured.