It is assumed that you have set-up a dual boot system. The boot screen
lists the Operating systems you have installed on your computer and it
is the screen that by using the arrow keys and enter key allows you to
choose what operating system you will be using.

If after installing Ubuntu or ??? distribution on your machine you fail
to get this screen on booting your computer. It is likely that the Linux
install was not successful. installing Grub is the last thing the Ubuntu
installer installs and it is dependent on a successful install. If the
install failed check the live CD with the check sum and try another install.