Openshot requires a number of codecs to function correctly - especially when it comes to creating DVDs- typically it needs mpeg and other similar codecs.
A codec is a program or code library that converts or translates from one format to another. There are numerous codecs for video and sound.
Whilst you can install these codecs manually, the easiest way is to install ubuntu-restricted-extras
This is a package that gathers together the more commonly known codecs and installs this onto your computer.
note - for some countries (notably the united states) - some of these codecs are restricted from usage by patents etc.. so you need to be aware of your country's laws etc for what codecs you can lawfully use and in what circumstances you can use them.
To install - look in software-center for ubuntu-restricted-extras or install from the command line:
sudo apt-get install ubuntu-restricted-extras
There are similar named restricted-extras - you should install the appropriate named package for your desktop environment. for Unity & Gnome-Shell use