This is the installation disc for Xubuntu 18.04 LTS (long term support) Bionic Beaver amd64 (64 bit).
Xubuntu is a Linux distribution which provides great performance, great customization, wonderful desktop styles, and that nice Ubuntu ease and feel.
Xubuntu is free and open source software (FOSS). This means you can use it for free, and you can even edit its source code.
Xubuntu is a fork of Ubuntu, meaning it is Ubuntu based. It is mainly Ubuntu, retaining the familiar apt package manager and compatibilities. Its differences start at its default window manager, which is Xfce. But it also packages other utilities like the simple but featured text editor Mousepad and the file manager Thunar.
This distribution is often used to "breath new life into old PCs." Meaning it is often used to provide a modern system that runs well on an old and under-performing system. And it certainly lends itself to such a task, Xubuntu is one of the lowest resource-impacting modern operating systems around, only being beaten by Lubuntu and headless operating systems in certain cases. But it certainly looks better than either.