Let’s introduce to asymmetric cryptography. In a public key infrastructure, a standard user owns 2 keys: one private and one public.
Let’s imagine a situation where 2 users, Bob and Alice, exchange by mail, but with security. Therefore, Bob ciphered his mail content with Alice public key.
Then he sends the ciphered mail to Alice through the information system:
Now Alice deciphered the mail sent by Bob using her own private key. It is why it is important she only possesses her private key for being the only person that could decipher message address to her, using Alice public key.
It is all related to the encryption mechanism 🙂