A private key acts as the password to your wallet. It allows you allows you to get access to your coins and spend them. When you control your private key, you can be sure that you will always have access to your fund. If you don’t own your private key you don’t own your coins.
The private key is not accidently called private key. Whoever knows your private key can get access to your wallet and remove your funds. Thus, it should never be shared with a third party, unless you want to give them access. If you lose your private key you lose your cryptocurrencies forever. For this reason, you must keep your private key safe.
Like public address, private key is also a very long alphanumeric string, 64 digit to be exact and it looks like this: 3a1076bf45ab87712ad64ccb3b10217737f7faacbf2872e88fdd9a537d8fe266.
There are also a human readable versions of private keys which are called seed. Seed a master private key and it is used to backup and restore a cryptocurrency wallet. Most wallets will ask you to write down a list of words (typically from 12-24). Later on if you lose your hardware wallet or mobile phone, your seed can be used to regenerate your private key so you get access to your cryptocurrencies.