First things first, you should NEVER share your private key with anyone else and you should never store your private key online.
Repeat it for the people in the back: You should NEVER share your private key with anyone else and you should never store your private key online.
The private key is crucial to keeping your coins safe. It's the counterpart to your public key that you can use publicly and is expressed at times as a wallet address. This is unique to you and only you should know/have access to your private key. It's an alphanumeric string that signs a digital communication once hashed with your public key. Dig deeper into the technical stuff here: https://blog.wetrust.io/why-do-i-need-a-public-and-private-key-on-the-blockchain-c2ea74a69e76
Another way that a private key is expressed is referred to as the recovery seed, a twelve to twenty four word phrase that allows you access your coins through any wallet or device.
Let’s say you have lost your your hardware wallet or your current wallet provider does not support a coin held by your address, but you have responsibly kept your backup seed phrase to restore the wallet. You'll find the recovery wallet option of the third party service you are using and the seed will allow access after entering the random words from your seed, the interface may ask for a password or pin and your new device/wallet should show your funds.