This document from Microsoft explains what DISM is. Be prepared for lots of long winded commands and names. You’ll be using DISM if you use Microsoft’s products to automate or customise Windows desktop or server deployments.
“Deployment Image Servicing and Management (DISM) is a command-line tool introduced for the Windows® 7 operating system that can be used to service a Windows image or to prepare a Windows Preinstallation Environment image. The functionality that was included in Package Manager (pkgmgr.exe), PEimg, and Intlcfg is now consolidated in DISM, and new functionalities have been added to improve the experience for offline servicing”.