A passphrase is something you know and is like a password except:
In the early days passwords on Unix system were limited to 8 characters, so the term passphrase for longer passwords.
Longer is the password harder it is to guess.
Nowadays Unix systems use MD5 hashes which have no limitation in length of the input (ie password).
cp server.key.pem server.key_with_pwd.pem
openssl rsa -in server.key_with_pwd.pem -out server.key.pem -passin file:pass.txt
You can create a key from a passphrase by using a password-based key derivation function (kdf)