UDF Functions

Dear experts,

I am a newbie to XI.Presently trying to understand UDFs.When i look at coding ,i could not conclude how developer

interpret that (x,y,z) parameters are passed to this function.

At what stage UDFs are called. ?.

Example

public void method (String [] p1 String [] p2 ResultList r1)
{
//write your code here
}

What is ResultList.I heard of ArrayList till now .Also i find some peculiar functions been used here example

public String StringToDecimal(String a,Container container) {
DEC b = Convert.ToDecimal(a) ;
return b;
}

I

Plz help.