Dynamic-link library (DLL) is a module that contains functions and data that can be used by another module (application or DLL). The term is usally applied to native DLL, the .NET DLLs are usually referred just as 'library' and are vary different in nature and usage.
I have lost my Unity project which was on my hard drive. Fortunately, I have found some files that were associated with my project. See here. I have successfully decompiled the DLL's to C# with a DLL ...