Paraphrased from Wikipedia:
Infrared sensing in snakes depends on a kind of natural thermography, by which tiny packets of cellular water are raised in temperature by the infrared radiation.
What is "packets" refering to in this is example?
This is probably a question for Biology.SE however the tiny packets are just that, very small sacks of water. Think tiny drops. There is so little water that it doesn't take much energy to heat it up via infrared radiation and the cell can detect the temperature change.