REDRUM (/ɹidː. ɹʌm/), also stylized as REDЯUM, refers to the word murder, spelled backwards.
If you haven't worked it out yet, Redrum is a palindrome of murder and after it's scrawled on the door in The Shining, Danny's mother notices that in the mirror, redrum actually reads murder. If Danny's mother hadn't woken up in the scene above, the young boy almost certainly would've killed her in The Shining.