FCReloadFunctionFromFile[function, path] is an auxiliary function that attempts to remove all the definitions of the given FeynCalc function and then reload them from the specified file.
It is intended to be a helper tool for FeynCalc developers, which allows one to debug/improve internal functions and test the results without restarting the kernel. Depending on the complexity of the given function, there might also be unknown side effects.
The function is not meant to be invoked by the normal users.