Do I need DRM on my ebook?

Generally this is not a question for book authors or self-publishers.

DRM is a thing for online bookstores, distributors or big publishers, such as Amazon, iBookstore, BN, etc. Each distributor has their own DRM, only for their devices or systems, to protect their copyrights.

For most authors and self-publishers, you will submit your books to these big distributors, and they will add DRM for your books before they are downloadable for readers.

If you sell your book on your own website, need not those distributors, you may still can not use DRM, because the cost is too high for person, you will have to apply a DRM from Adobe.