How do macros work ?

Context: I'm in the market for a mechanical keyboard and I fell in love with Razer's wristrest. Currently hesitating between the BlackWidow V2 and the Elite. I'm trying to understand how the macros work on the Razer keyboards (on the ones that don't have dedicated macro keys). I understand I should be able to assign a macro to ANY key.....but then that key loses its primary function or will I have to press Fn+Key or something like that? I expect to use macros in Windows and MS Office rather than games so I will need to keep the primary function of the key as well. By the way, I did RTFM, to no avail: https://dl.razer.com/master-guides/RazerSynapse3/BlackWidowElite-00000552-en.pdf