A password is simply an access code or a pass code. Volumes have been written about the dangers of password reuse and of using weak passwords. Statistically, the leading cause of computers being hacked is password reuse and/or the use of weak passwords.
To reset a forgotten password, boot your computer from a life-CD like Knoppix. Edit /etc/shadow, you will find lines like the following:
here, "myuser" is the username. Behind it is a colon and the encrypted password. Reset the password by removing the complete line. Reboot your computer - your password is empty, you should be able to log in and set it using passwd