You may choose to enable Digital Rights Management (DRM) when you publish a book on Kindle, but we do not recommend it.
Once you make the choice, you cannot change it. Most successful authors do not recommend enabling it. DRM is software that attempts to limit piracy, but is not always successful, and can cause problems for honest customers. There is a consumer backlash and grassroots movement against DRM.
This information and more can be found in the resource section of How to Self-Publish: A Guide for Author-Preneurs, 2nd edition by Kayla Fioravanti.