Figuratively it refers to a well known
truth behind some concept (for example, "santa isnt real") that we collectively decide to ignore, because it ruins the
fun.
It's in reference to mascot costumes worn by some poor soul to appease the masses, for example the underpaid part-timers staffing the costumes at disneyland
It's
like, as a kid, pulling the "no, you didint just kill me cause we are just pretending" card when playing pretend on a playground
Alternatively,
man behind the mask
Weeb A: you do know that most weebs are ironic about waifus, right? Only a relatively small amount of weebs actually think
stuff like "this fictional high schooler is the love of my life".
Weeb B: Can we please stop talking about the
man in the costume and get back to arguing about which underage girl is hotter?