What is "Strong Password" requirements?

Looking in the docs, for local accounts, you can enforce "Strong Password" requirements. In the documentation, it doesn't specify what "strong" is. Anyone know specifics? 

https://docs.appdynamics.com/display/PRO41/Users+and+Groups#UsersandGroups-RequireStrongUserPasswords

What I think is a strong password is probably more than what others may think. Anyone know for sure? 

Thanks,

Colin