The Reality Checkpoint is an ordinary lamppost that stands at the centre of Parker's Piece, a large open common on the edge of central Cambridge where town and gown famously intermix. The lamppost acquired its informal name as a student in-joke about the transition between the enclosed academic world of the university and the ordinary world beyond it. The name is now painted on the base.
Parker's Piece itself is worth knowing about independently — a large flat common where local football and cricket have been played since at least the 16th century. The 1848 Cambridge Rules for association football were developed partly on Parker's Piece; the modern laws of football trace a direct line from those rules.