DLL stands for Dynamic Link Library, is Microsoft’s way of shared libraries, allowing multiple programs or applications to use same file or its functionality. Files such as msvcrp100.dll are used by various installed programs or built-in Windows programs to share functionality amongst themselves without the need to have duplicate files.
DLLs enable developers the ability for shared code and data, to upgrade functionality and features, and saving the need to re-link or re-compile applications.
Initially, Windows allowed programs to operate together in a single address space. Programs were designed to co-operate by sharing the CPU with other programs, kind of enabling Windows to multi-task and be responsively lightweight.
E.g. Suppose you play a song on your VLC media player, VLC will use the .DLL file associated with audio-playing functionality. The same DLL file that is used for audio-playing will be used when you use any other media player, to play music.
This was just an example; I’m neither implying nor saying that audio-playing functions are based on DLL files.