Mrs LB:
An 'operating system' is the computer coding which is on a computer (or, say, a mobile device, such as a phone), which turns it from a set of electronic components into a working device.
[A 'regular' PC normally uses Microsoft Windows as its operating system but some people prefer to use Apple devices, which use macOS instead. Mobile phones though use Android for their operating system, unless one buys an iPhone, which uses iOS].
However, while the makers of 'regular' PCs normally put Microsoft Windows onto to them (in order to get them to work), there's another operating system, called Linux, which can also be used on them. If you were to plug an Xtra-PC memory stick into a USB port on your computer, and then turn it on, your computer would (as long as it was set up to do so) completely ignore the fact that Windows is on its hard drive and use Linux from the pen drive instead.
However, unlike Microsoft Windows, Linux is completely free. It's illegal to charge for it but it's OK to put it onto a memory stick and then charge for the stick (plus a small fee for the task of copying Linux onto the stick). So the people selling Xtra-PC are actually charging €33.50 for a memory stick, that's worth no more than a few euros, and putting some free software on it.
Anyone who wants to try Linux, in a hassle-free way, can buy a USB memory stick with Linux on it from eBay for far less than €33.50. Linux actually comes in loads of different versions though, such as Ubuntu, Debian, Linux Mint and Puppy Linux. (Some of those have sub-versions too, such as the various 'flavours' of Puppy Linux). The memory stick in my eBay link, above, has more than two dozen different types of Linux on it, which a user can try at their leisure. (If they really like a particular version, they can then permanently install into onto their computer's hard drive, or SSD, either alongside Windows or instead of it). The Xtra-PC memory stick only has one version of Linux on it and costs a lot more!