GnuPG is a publicly available encryption scheme that uses the "public key" approach. With GnuPG, messages are encrypted using a "public key" however, they can only be decrypted by a "private key", which is retained by the intended recipient of the message.
You can also import an existing key by clicking the "Import Key" button at the bottom of the same page. Then simply copy the key code you want, and paste it into the text area provided. Click the "Import" button, and the key will be added to your account.
You keep your private key secret from everyone, but you give your public key to anyone who wants it.
If you want to send someone an encrypted email, you need to get their public key and use that to perform the encryption. The receiver must use their own private key to decrypt the email.
If you want someone to send you an encrypted email, you need to give them your public key. Once they send the message, you must use your private key to read it.