The source for this halakhah is the passage in Sefer Bamidbar (31:23) that describes how after the war with the Midianites, all metal vessels that were taken as booty in the war needed to be washed be-mei niddah – in a kosher mikvehcontaining 40 se’ah of water. This is not a requirement connected to the laws of kashrut; the Torah requires this even if the utensil had been made kosher by means of heat in fire beforehand.
Rav Nahman quotes Rabbah bar Avuha as teaching that this applies not only to used utensils, but to new ones as well, assuming that they are actually purchased and owned by the Jew, but not if they were simply borrowed. Rav Sheshetchallenges this teaching by suggesting that all utensils should require this dipping in a mikveh – even a sarbela – a scissors. Rav Nahman responded that the case in the Torah dealt specifically with utensils used for food and food preparation, so this law is limited to those situations.
The Talmud Yerushalmi teaches that the reason for this law is because the utensil is moving from an ordinary existence to the higher level holiness of Judaism; the Ritva compares this dipping in the mikveh to a case of a ger – a convert – whose change of status to a Jew is confirmed by dipping in the mikveh. Limiting the requirement to food utensils indicates that kosher food preparation must concern itself not only with the technical kashrut of the food, but to a higher spiritual level of the food, as well.