Also called: tuple slot, result slot, scan slot
In plain English
A TupleTableSlot is the small holder that carries the current row between executor nodes. It can point straight at a tuple still sitting on a disk page, or hold a freshly built row in memory, without forcing a copy each time a node hands a row upward.
Why it matters
Slots are why the one-tuple-at-a-time model stays cheap: passing a row up the tree is passing a slot, not copying bytes. They also defer the expensive step of splitting a stored tuple into individual columns until a node actually needs a column — a key performance trick.