Google's Chrome OS can now install and run Debian Linux packages with the .deb extension, at least in the Canary channel. This happens by simply double-clicking the file in your file browser. From there, the automated installer takes over. Once a Linux application is installed, it will be available in your terminal, invoked in the same way as a Chrome OS app, and some apps may show up in your Chrome OS launcher, depending on the metadata present in them and whether they support such operations. Most Linux apps that have a graphical user interface fall into this category, though there are many command line utilities out there for Linux users to enjoy. Both are now available to Chrome OS users. You still cannot replace default Chrome OS utilities, so don't expect to run an i3 desktop with a brand new ALSA media handler unless you're willing to gut your Chromebook entirely and install Linux.
This functionality has been around in Chrome OS Canary for several build versions now, but up until the most recent update, has not been officially accessible. It required a number of hacky workarounds, and driver support, among other things, was sorely lacking. The installer and the ability to run Linux applications is now in a fully functional state, though you have to keep in mind that you'll likely get better performance from a native Linux installation. You shouldn't expect GIMP to load up its plugins and have you ready to edit an image in seconds, nor should you expect just any SteamOS-compatible game to work with no issues out of the box on your Chromebook. You'll also have to hop into a terminal and use the "sudo apt update" and "sudo apt upgrade" commands before you begin, in order to get the Linux package manager up and working.
The new functionality should be available on any Chrome OS device that currently has support for Google's Crostini project, the company's attempt to integrate Linux kernel codebases and functionality into Chrome OS. If you're not currently on the Canary build of Chrome OS, keep in mind that enabling development support and going over to a new channel involves wiping all of your Chromebook's storage and user data, so make a backup beforehand if you have anything worth preserving on your device.