“”Who controls the past, controls the future: who controls the present controls the past.
|—1984, George Orwell|
Memory hole is a neologism that refers to... uhh...
The term comes from George Orwell's famous novel 1984, where "memory hole" referred to a file transfer system that led to an incinerator. Since all texts contradicting the Party's take on history were disposed of, citizens had no evidence that the government was wrong. So the government could actually rewrite history however it wanted.
Today the term is used metaphorically for when people (most often politicians) carry on as though a certain event in the past lacks historicity. An example would be attempts to gloss over the 3/5 Compromise or ignore certain past atrocities like the Holodomor; a more modern example would be Ben Carson on his supplement-peddling past, and the general unwillingness to call him out on it.