Receptors are born in the cell. The cell's DNA contains the instructions to create each type of receptor. The cell runs the DNA program, which results in the creation of a receptor protein, and that receptor protein is then pushed out to the cell boundary and embedded in the cell's membrane. This is called ‘expression' of the receptor. Once receptors have been expressed, the cell can receive and respond to messages the receptor is tuned to.
But the receptors have a life cycle.
They can be taken out of the membrane and recycled into the cell. So fewer receptors are left on the cell membrane and therefore the less notice is being given to that specific message. So a decrease in the number of receptors to a message sited on the cell membrane reduces the cell's sensitivity to the message.
That's called down-regulation.