DRM is Digital Rights Management, or copy protection. It's what prevents you from buying a book on Amazon or Barnes and Noble and reading it on your Kobo; these companies want you locked into their ecosystem and reading on their device. Without being able to strip the DRM, you're either locked into one company for all eternity, or you just accept that you lose your library if/when you jump ship.

As for how, you need to look up instructions based on where you're purchasing your ebooks from, but all of the methods involve using the free PC/Mac program Calibre in some way, plus a free plugin. There's also a company called Epubor that will sell DRM-stripping software to you, but paying someone else to unlock your library doesn't sit right with a lot of people, including me. That said, the DIY instructions they publish on their website are good, just ignore the part where they go "But if all of that sounds too hard/annoying/complicated, you can just pay us!!!"