root?

[dmg1287]

When I try to do something it says I must be root to run this application, what does that mean?

[Lakshmipathi]

When I try to do something it says I must be root to run this application, what does that mean?

root refers to super user (like administator in windows) - when normal user try to access/execute root user (admin) commands - It will either say
" command not found" or "you must be root to run this application".

Some basic question -answered here
The Fedora Basics FAQ

[dmg1287]

okay thanks but when do what it says to become root it says su: authentication failure, So what do I do?

[egan]

That means that for some reason, access was denied (based on your privileges or a failed password). Am I right to assume this is a personal PC? If it isn't, and you aren't the administrator, then you may have not been given the right to super user access.

Also, it is probably better to use sudo rather than su.

[bigtomrodney]

okay thanks but when do what it says to become root it says su: authentication failure, So what do I do?

You are using Ubuntu and in this particular distro direct root logon is not permitted by default.

What you must do instead is either prefix the command with sudo which runs that one command as root, or run

sudo -i

first to give you a root shell.

HOWEVER....if you are running a graphical application do not use sudo, but rather gksu which is designed primarily for graphical applications. Can I ask what it is you are trying to do?