It's the end of the road for Windows 7. After 10 years of support, Microsoft is ending security updates for the operating system today. This means that if you're still using it you'll be vulnerable to bugs, and any problems that arise won't be fixed (unless you're part of a company that's paid for extended support, in which case you're good until 2023).
So to make sure you've got adequate protection, you need to upgrade your OS. Back in 2015 Microsoft made upgrades from Windows 7 or Windows 8.1 free for the first year, but while it officially ended the Windows 10 free upgrade offer in December 2017 there is a loophole that means you can still get your hands on it for nothing -- providing you have a legit Windows 7 or 8.1 license.
As reported by Windows Latest, users can download and run the Media Creation Tool, and then enter their existing product key if prompted. Once upgraded, you'll have a digital licence to Windows 10 that will be verified when you connect to the internet. Over on Reddit, a self-proclaimed Microsoft engineer stated that the original free upgrade deal was "fully marketing fluff," and that Microsoft would rather folk upgrade via the loophole than not at all, because then they'll at least have an up-to-date OS -- and subsequently, better protection.
So it seems that Microsoft is not particularly troubled by the loophole. However, as Windows Latest reiterates, a successful activation of Windows 10 is not the same as a legal license, so you could technically be violating licensing agreements by using a Windows 7 product key for Windows 10.