What's a receptor?

re·cep·tor
rēˈseptər/
nounPhysiology
noun: receptor; plural noun: receptors

an organ or cell able to respond to light, heat, or other external stimulus and transmit a signal to a sensory nerve.
a region of tissue, or a molecule in a cell membrane, that responds specifically to a particular neurotransmitter, hormone, antigen, or other substance.

they're talking about molecular mechanisms embedded in the surfaces of cells. specifically they're protein molecules. parts of them stick out from the outside of the cell.

these protein molecules have the property that when some specific molecule from outside floats up to the parts outside, they're shaped just right to bind to those parts, and when they do, the addition of the new molecule causes the protein molecule to change it's shape, in just the right way so that the new molecule is pulled into the cell and is exposed on the cell wall's inner surface instead of the outer surface.

at that point other molecules inside the call grab onto the transferred molecule and pull it away. once it's removed, the receptor molecule goes back to it's original form and waits for another customer

they're used in cells all over the body, whenever a cell needs to pass specific molecules and reject others, across a cell wall