You are probably reading this because you saw multiple processes with name rundll.exe in your Windows task manager and you are wondering what does it do.
Since you are a Windows user, you must have seen .dll files on your computer. The .dll files, or dynamic link library files, are library files that store the executable code which is used by multiple applications. If you are familiar with programming, then you can think that they store functions that can be called from other applications.
The internal structure of a .dll file is same as that of an executable file (.exe file) but Windows cannot run or execute a .dll file directly. So, if there is a need to run a .dll file, rundll32.exe is used to do that.
So, if you see multiple processes with name rundll32.exe running at the same time, then it probably means that multiple .dll files are being executed at that time.