unable to authenticate...

It depends on what the "something" that you are trying to do is. Most likely, the reason it does this is because it is not trying to log on as *your user*, but is instead attempting to perform some system-wide action.

Allowing this without password authentication would be a fairly hefty security flaw, as any application could immediately promote itself to administrator mode, and do whatever it liked to your system - even without your consent. Remote attack vectors in particular would be very useful in this mode.

Even windows, which traditionally used the "no password" model has moved away from the "run everything as admin" mode (UAC), as this idea is fundamentally problematic in computing.

...
But now which password should i provide to it????????????

When you installed your system, you would have been asked to create a password, regardless of whether you specified that you wanted to auto-login or not.

You can reset the password by following this procedure:
https://help.ubuntu.com/community/LostPassword

i had given the same password for few times but still it shows same that authentication failed....

while searching i came across this...

Choose “Login without a password” in first drop-down box and click “Change” at bottom.
Now, you can log-in this user without password both in GUI log-in screen and text (tty1~tty6) mode. The password is actually set to “None”. So in the tty1~tty6 (press Ctrl+Alt+F1~F6), it won’t ask a password for running commands with sudo at beginning. But in desktop environment, sudo command and “Authentication Required” window still need password and you can’t get through whatever you input. To deal with that see following steps.
2.) Reset password in command-line. After that, you can login without password in LightDM Login Screen and all others back to normal.
Press Ctrl+Alt+F1 to navigate to TTY1 and log-in with your username. Then run this command to reset password:
sudo passwd YOUR-USER-NAME
Then input a new password and again to confirm.

(bit confused....)

Thanks Sam_, that solved my question.

i am gREATFUl to u n ubuntu LAUNCHPAD...
once again thanks alot!!!!!!!!!!!!!!!!!!!!!!
:)))))