If you have dual-boot system, aka. you have 2 (or even more) operating systems, you have to select at startup which one to use. Now if you had windows on your machine before and installed Ubuntu afterwards, Ubuntu will be the default OS to start in the GRUB boot manager. This can be changed of course.
I have both Windows and Ubuntu on my desktop computer, but I am using Windows more often so I wanted to have that started by default and only start Ubuntu when I need it. Here is how to change which Operating System to boot automatically.
Change GRUB default boot order
First boot into Ubuntu, open a terminal and type:
sudo gedit /etc/default/grub
Look for the GRUB DEFAULT parameter. This is set to 0 by default, which represents the first element in the boot menu list, hence the formula is parameter = (list element) - 1
Explained as below:
GRUB DEFAULT set to 0 = 1st entry in the list will start by default
GRUB DEFAULT set to 1 = 2nd entry in the list will start by default
GRUB DEFAULT set to 2 = 3rd entry in the list will start by default
and so on.
On my installation Windows was the 6th in the list so I had to set the value to 5.
That is it done. Sign out and restart your system to see the result for yourself.