What is a public/private key pair?

A secret private key you keep hidden plus a public key mathematically derived from it and shared freely; together they let you prove ownership without revealing the secret.

The relationship runs only one way: easy to go from private to public, infeasible to reverse. Lose control of the private key and someone can act as you; lose it entirely and you lose the account forever.

Go deeper:

• Public-key cryptography — Wikipedia — diagrams key generation, signing, and encryption with a key pair.