Forget your passphrase don't worry. Powershell will come to your rescue.
Why we need passphrase, it is required when we are adding a new server to our farm.
If you don't know it then you cannot add server to your farm
Below PowerShell can help you setting up a new passphrase for your farm.
$passpharse=ConvertTo-SecureString - String "SP2010Password" -asPlainText -Force
Set-SPPassPhrase -PassPhrase $passphrase -Confirm
Thanks to Stewart Wainaina for sharing this
Happy SharePointing :)