Thinking to install Chrome to expedite signing in to Gmail and Google Calendar every day at work, I see in small text near the bottom of the page that I would have to 'add the Google repository to my system'. What does this mean? Would I be installing many unnecessary files, bloating the computer? Or would only links ('paths'?) be added to the update software listings to check for new packages when it routinely checks Ubuntu repositories for updates? In other words, only a few kilobytes of changes to my computer?
Ubuntu itself comes with tens of thousands of software packages which you can install. A default Ubuntu install will only include a set of default packages. For the others, you download and install them using the package management system built into Ubuntu. The part of this system that downloads packages from Ubuntu's servers is called APT.
In a default install APT is set to only download packages from Ubuntu's own servers, also referred to as Ubuntu's software repository. While this contains a huge range of software, some software cannot legally be provided by Ubuntu because it has a proprietary license, so it isn't included in Ubuntu's sortware repository. This includes the official Google-branded "Google Chrome". While there is a non-Google branded version called "Chromium" that may be distributed as open source software, it is not Google's official version and does not have Google in the name.
Other companies can set up their own software repositories allowing Linux users to install their software even when the software isn't available through their own operating system's software repositories. This is the case with Google, who provide software repositories for Ubuntu among other operating systems. To use the official Google Chrome browser with Linux you have to obtain it from Google's software repositories. Since Chrome needs regular updates for continued secure operation, this can't just be a once-off.
To use a software repository other than Ubuntu's own, you have to configure APT to use that repository in addition to the ones it knows about already. This is traditionally done by editing
/etc/apt/sources.list though there are alternative methods such as:
The PPA (personal package archives) system, an easy way for people to publish their APT software repositories on launchpad.net, also provides its own way to add those repositories to users' APT configuration.
In some cases the third party company will provide a setup script which basically just automates the process of adding their repository to your APT configuration so you don't have to do it manually.
Be careful of who you trust to add their repository to your system.
Ubuntu takes some level of responsibility for all of the software they provide, but third party repositories are not vetted by Ubuntu and their software is not checked for security issues. You must only install third-party software if it's from a source you trust, and you can verify it genuinely came from that source. Don't add lines to your sources.list or run scripts you see on online communities, go to the official source.