If you have installed Ubuntu or some other Ubuntu variants like Kubuntu, Lubuntu, etc, I think the first obvious problem you will notice is you can't play some audio or video files.
It's obvious, there are some media codecs that are absent, but don't panic, there's a package that provides all the essential media codes you'll need to play media. (You can read about it here)
Run the following command to install this package.
sudo apt install ubuntu-restricted-extras
Alright, that should permit you to play most videos. Audios should work just fine.
For some videos, it doesn't. The above package doesn't support AV1 media types. To solve this, run the following command.
sudo apt install libdvdnav4 libdvd-pkg \ gstreamer1.0-plugins-bad gstreamer1.0-plugins-ugly libdvd-pkg
Note: These packages are not provided out of the box because they are not open-sourced. So During the installation, you might be prompted to accept licenses. (EULA, I think 🤔)