When Passmass runs, it asks you for the old and new passwords. (If you are changing root passwords and have equivalencing, the old password is not used and may be omitted.)
Passmass understands the "usual" conventions. Additional arguments may be used for tuning. They affect all hosts which follow until another argument overrides it. For example, if you are known as "libes" on host1 and host2, but "don" on host3, you would say:
passmass host1 host2 -user don host3
Next argument is a program to run to set the password. Default is "passwd". Other common choices are "yppasswd" and "set passwd" (e.g., VMS hosts). A program name such as "password fred" can be used to create entries for new accounts (when run as root).
Next argument is 1 or 0. If 1, you are additionally prompted for a root password which is used to su after logging in. root's password is changed rather than the user's. This is useful for hosts which do not allow root to log in.
Using the same password on multiple hosts carries risks. In particular, if the password can be stolen, then all of your accounts are at risk. Thus, you should not use Passmass in situations where your password is visible, such as across a network which hackers are known to eavesdrop.
On the other hand, if you have enough accounts with different passwords, you may end up writing them down somewhere - and that can be a security problem. Funny story: my college roommate had an 11"x13" piece of paper on which he had listed accounts and passwords all across the Internet. This was several years worth of careful work and he carried it with him everywhere he went. Well one day, he forgot to remove it from his jeans, and we found a perfectly blank sheet of paper when we took out the wash the following day!