If you need to mass reset the password for some of your Drupal users, drush can help you.
The drush command to reset a user's password is:
drush upwd username --password="newpassword"
Now, let's say you need to reset 150 user's passwords.
Create a file named accounts.txt with all usernames you want to reset (one per line)
Run the following command:
while read line; do drush upwd "$line" --password="newpassword"; done < accounts.txt