In the highly unlikely event that you ever, for whatever reason forget your password to log into Plesk, one way of recovering the password is by loging into the server via SSH.
In order to view the contents of the file that contains the admin password you will need to be able to acquire root. Once root, you just simplely need to look at the /etc/psa/.psa.shadow file.
# cat /etc/psa/.psa.shadow The$up3r$ecretPa$$w0rd