Without getting into semantic arguments about kernel-vs-OS…
It’s a system that does the same kind of things as Windows or Mac OS, such as managing programs and hardware, and presenting a desktop, taskbars, etc, but it does it in a different way from how they do.
It was mostly written by programmers For programmers, so there are a lot of rough edges still. also most “Distributions” of Linux don’t cost anything, but some charge for support or custom utilities. It’s not true that you can’t profit from it. (it’s just hard to)
I love it myself because it’s so possible to modify (even without coding) but it can take a Lot of effort to get it working, and working the way you want (if you aren’t lucky.) I have long claimed that the attribute you need most to get much out of it is not intelligence but stubbornness (and skill at Google)
Edit: I should also mention that once it is working, it can be very easy to use. You still have access to programs like Firefox for example. I have set it up for people not particularly computer-savvy and they have had few problems, and none of the viruses/trojans and such that they got from Windows.