What is a memory hole in 1984?
The memory hole in 1984 is an opening in a wall connected to a chute. Anything no longer deemed to be of use is put into the memory holes and subsequently incinerated. They are primarily used to destroy unwanted documents, and Winston Smith, in his job at the Ministry of Truth, routinely throws documents into the memory hole that show how history has been tampered with by the Party.
Memory holes can be seen as crude garbage disposal units. They are holes in the wall connected to a chute down which unwanted stuff is sent to an incinerator. The primary use of memory holes in Oceania is to dispose of documents, especially historical documents such as photos and old newspaper reports. It's not enough that the Party controls the present and the future; it also has to control the past. And it does this by rewriting history to follow the ever-changing Party line.
Winston Smith's job at the Ministry of Truth regularly involves the use of a memory hole. As part of his work, he receives original documents that he is required to alter according to strict instructions. Once he has rewritten history according to the Party's requirements, the original documents are then shoved into the memory hole and subsequently incinerated. It's as if the past never happened.
The memory hole thus becomes a symbol of how thoroughly this totalitarian regime controls the past. In Oceania, there is no objective truth; the truth is only what the Party says it is. If the Party says that Oceania has always been at war with Eastasia, even if previous government documents said the exact opposite, then Oceania has always been at war with Eastasia and that's that. All documentation relating to the previous situation is altered to reflect the Party's changing position, with the original documents being sent down the memory hole.