Private Key
In short: A private key is a secret key granting access to funds on a blockchain. It is a very large number, usually shown as a long string of letters and digits…
A private key is a secret key granting access to funds on a blockchain. It is a very large number, usually shown as a long string of letters and digits, that proves ownership of an address and is used to sign every transaction sent from it.
Why it matters
Whoever holds the private key controls the funds, and a blockchain has no password reset or support desk: a lost key means lost funds, and a stolen key lets someone else move them. Your public address is derived from the private key, but the key cannot be worked out from the address, which is why you can share the address freely and must never share the key. Most modern wallets derive their keys from a seed phrase. On a custodial exchange, the platform holds the keys to the deposit wallets on behalf of its users.
Example
When you send coins from a self-custody wallet, the wallet builds the transaction and signs it with your private key. The network checks the signature against your address and accepts the transfer without ever seeing the key itself. If a scammer tricks someone into typing their private key into a fake website, the scammer can sign transactions too and empty the wallet.