The sources.list contains a list of the sources that can be used for the installation and update of software packages.
If you installed your Debian Linux distribution from a CD/DVD at some point, or maybe just by default if there is a CD/DVD player in your PC, an entry will have been added to the sources.list file corresponding to the mount point of the installation CD/DVD.
Usually you can deactivate this entry after the initial installation, as I suppose that you further installs and updates will be from network sources.
The full path to the location of the file is /etc/apt/sources.list. You can edit the file in a text editor, but as a newbie it is probably easier and safer to do that via your graphical update/package manager.
Have fun discovering Linux.
The sources.list contains a list of the sources that can be used for the installation and update of software packages. If you installed your Debian Linux distribution from a CD/DVD at some point, or maybe just by default if there is a CD/DVD player in your PC, an entry will have been added to the sources.list file corresponding to the mount point of the installation CD/DVD. Usually you can deactivate this entry after the initial installation, as I suppose that you further installs and updates will be from network sources. The full path to the location of the file is /etc/apt/sources.list. You can edit the file in a text editor, but as a newbie it is probably easier and safer to do that via your graphical update/package manager. Have fun discovering Linux.
For more information, have a look at: https://wiki.debian.org/SourcesList