Instacure #2 is a curing salt comprised of roughly (depending on the brand you purchase) 94% salt, 5% sodium nitrate and 1% sodium nitrite.
Instacure #2 is also known as Prague Powder II.
It is used specifically for making dry cured products such as hard salami, pepperoni, and prosciutto. It works well with meats that are aged a long time, because the sodium nitrate will break down over time and convert to sodium nitrite, then nitric oxide to cure the meat.
This is strong stuff - make sure you wear food preparation gloves when you handle it.