Glossary · Wallets
Private key.
The secret that authorises spending from an address.
Think of it as
The signature only you can make. Keep it, you own the wallet; lose it, the funds are gone for good.
In depth
A private key is a long random number that only the wallet owner should ever see. Cardano addresses include a hash of the matching public key; spending a UTxO requires a signature produced by the private key. Lose it and the funds are unrecoverable; leak it and anyone can spend.
Example
When you "sign" a transaction in Lace, the wallet uses your private key (still inside the wallet) to produce a signature that the network can verify against your public key.