If you use a Debian-based Linux distribution then you probably use Synaptic. And if you use Synaptic enough you may have come across an application that it can not find. More here
When you find that combination you will need to add repositories to Synaptic that will allow the system to find and install the application you are looking for. Fortunately you do not have to do any hand editing of a text file (you can if you want’…but you don’t HAVE to.) Instead you can use the GUI application Synaptic to handle this task.