DRM stands for Digital Rights Management – the controls that you (or an ebook vendor) place on your content regarding copying, printing, and sharing. Traditional publishers encode their ebooks with DRM to prevent piracy of the file.
The debate as to whether or not this actually works is heated and ongoing. But as a self-published author, you’ll have to decide if you’re going to put DRM on your ebook files or not.
Proponents of DRM view it as a necessary evil. While DRM can certainly be hacked, it does prevent the average user from giving away your ebook file to someone else, or copying it and uploading it to a piracy website. You can also prevent people from printing out the file.
Those opposed to DRM view it as essentially pointless. Yes, it can be hacked. But downloading hacked ebooks is actually a bit complicated and the average user probably isn’t going to find it worth the trouble. DRM also limits gifting or lending, the way we do with physical books after we’ve bought them.
Ultimately, the ebook vendor may put its own DRM on your file, for its own reasons. But in most vendor environments, the publisher can choose whether or not to protect files with DRM.