What does the automatic login option do?

Under "User Accounts", there's a toggle option labeled Automatic Login:

What does that do if it's tabbed to "on"?

I haven't tried the function yet and I'd like advice from an expert.

Solution 1:

"Automatic Login" Inside of each User's options on the User Accounts dialog (remember that you can have several users in the same computer), just means that the user won't be prompted for his/her user password in order to start his/her session.

You can have only one "Automatic Login" even if this seems to appear in each user. But even if you allow a user to boot without authenticating (signing in), the user may need to unlock his/her keyring for administrative purposes and will always be in need to authenticate each time he/she's in need to run a command with sudo or gksudo from a terminal.

If you need further information don't hesitate to drop a comment in order to continue the support and edit this answer if needed.

As mentioned by @Eliah Kagan in a comment to this answer:

If you log out to get to the login screen, and want to log back in, you still need to enter a password, even if automatic login is enabled. This is the case on every Ubuntu system on which I've ever used automatic login (which includes 13.04).

Solution 2:

the automatic log in means that the user will be logged in automatically and wont have to enter password and username at the log in screen.

That's it.