Rundll32.exe what is it?

Hello there curious what is rundll32.exe? I know that it is normal, but what does it do and why is there one in my SysWOW64 folder as well at the system 32 folder. I know it is supposed to be in the system 32 folder, but there was another one now on my task manager in the SySWOW64 folder. I am just curious if I am alright, and I am just being a little nervous for no reason. My computer does run fine though, but if I can get some help figuring out what this is, because I also don't know what my SysWOW64 folder is as well.