For taking the census of the Jewish people in the Wilderness each individual was to give a MACH’TZIT HASHEKEL, half of the monetary unit that was prevalent at the time. These coins were to be counted and hence the total number of coins would indicate what the count of the people was.
It is strange that Hashem did not just tell Moshe that everyone should give a half Shekel but started his instruction to Moshe by saying ZE YITNU, “This shall they give…”(Ex. 30,13) Because of these superfluous word our Chazal tell us the Moshe was not familiar with this coin and Hashem had to show him what the coin looked like. They tell us that Hashem showed Moshe a “fiery coin whose weight was a half Shekel”.
We can understand that it was necessary to show Moshe a coin but why do Chazal say that it was a fiery coin? A Rabbi gave a very insightful explanation. A half Shekel did not amount to much in value. The important aspect of the giving was that it should be fiery, that is, it should be given with a burning zeal. When participating in the census, each individual should do so with passion, with conviction and with determination. He is not only interested in being counted but wanted to be a zealous part of the people.