Hello, can anyone tell me how and for what this function works?

In a custom method with a list as parameter ,i am trying to do the next:
For each Object in list -> Object set property [CustomProperty] to [value]
For this i was wondering if i could use the function "[Object]
's Variable named [variableName]
of type [valueClass]" found in Object as "other" functions.
Like the function [Object]'s part named [key]. But my problem is i can't drag it anywhere. Where should it be placed?
I am really stuck at Generics Methods with this, because i can't set an object custom property.
Hope my english was good.