Simplified, the private key works similarly to how your password works when you are logging in to your email account.
However, that's the only intersection where the similarity meets and ends.
A private key is a string of random hexadecimals (numbers 0 to 9 and letters A to F) that looks quite similar to a normal wallet address. This is created concurrently with a wallet address' creation and is usually given to you by your wallet service provider when you create a wallet address. However, depending on the wallet interface that you are using, you may or may not be given access to the address' private key (please refer to your wallet service provider for more information on this).
The function of a private key is comparable to the password of your email account wherein everyone can see your email address but no one would be able to login to the email address without the password. Similarly, everyone can see your wallet address but no one can access the funds inside it without its private key.
The private key is neither non-renewable nor is it reproducible. You cannot renew your private key like you could change your email password and you cannot ask for a new private key for your wallet address if you have lost the original one. Each private key is unique to its own public key and is irreplaceable, in the sense that a private key can only be used to access only the wallet address that it belongs to and no two (or more) private keys are identical or the same. With the above being said, do take note that losing your private key essentially means losing ALL of the assets in your wallet address.
Your private key should always be kept, as the name suggests, privately. The most recommended and agreed upon way of storing a private key is by writing it down on a piece of paper and then keeping it somewhere that is private, safe and known only to you. You should never expose or share your private key to anyone, be it online or offline.
But what should you do if you have indeed lost the private key for your wallet address? Refer to our article here.