.deb files are simply Debian package files, Ubuntu also uses this type of packages. If you double click on a .deb file in Ubuntu a package installer will start and will install the package.
So if you download a .deb file containing Vlc, you just need to copy that .deb file to you ubuntu computer and double click it. It is possible that Vlc will need programs or libraries that are in other .deb packages. These are called "dependencies for Vlc" as Vlc depends on them in order to work properly. So you will also need the .deb files that contain those dependencies.
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