The MAGA on Donald Trump’s red hats stands for “Make America Great Again.”
It is a national Story which Trump projected in his presidential campaigns, and which inspired his Inaugural Address in 2017. It is false in the sense that it claims America is no longer “great” (whatever “great” means) because someone (unnamed) has caused the country to decline (in an undefined way). Since the guilty party is not named in the story, listeners can fill in the blanks with whoever they dislike.
In short, MAGA is an example of the false stories that help to shape post-truth American politics.