windowscentral[.]com/how-use-dism-command-line-utility-repair-windows-10-image
This article is easier to read & understand compared to what you find on Microsoft's site.
All of the files to install Windows are in a special type of image file called .wim that was developed by Microsoft. The install.wim file that holds all the files for Win10 is usually added to a Recovery partition when Win10's installed, and it's also found on a Win10 setup disc/ISO. DISM is a software tool to work with that install.wim file. If/when you have a problem with Win10, DISM can be used to make sure that all of the Win10 system files are as they should be. If you Google on a problem you're having with Win10, often the first thing you'll read is to check the Windows files using DISM, especially in the Microsoft forums, where it seems the standard boilerplate reply.