According to Microsoft, a callback function is code within a managed application that helps an unmanaged DLL function complete a task. Calls to a callback function pass indirectly from a managed application, through a DLL function, and back to the managed implementation. This repository contains a list of callback functions that can be used to execute position independent shellcode so that CreateThread would be a thing of the past :P.
Shoutout to alfarom256 and ch3rn0byl yall are fam.