It is a shared library concept, where a programmer can call the desired class form a DLL without having to code it by himself.
How to use DLLs in untiy C#?
Using DLLs in unity is a very simple process.
Create a folder called "Plugins"
Drag and Drop the desired DLL file in there
Reference the DLL from within your script
Example:
Adding DLL RedstoneinventeGameStudio.dll
Referencing RedstoneinventeGameStudio using the using keyword
Limitation of use of DLLs found on this page?
Every DLLs that we release have their own usage policy. When downloading a DLL file, there should be a EULA or ReadMe file inside the zip folder. Inside this, you may find the limitation of use for this specific DLL.