IC days is a formula for accurately determining the total cost of unit production. This formula provides data required in order to make long term production planning decisions.
IC cost * build time = IC days
For example, 1936 Infantry (assuming no slider bonuses) costs 7 IC to produce, and will require 95 days to complete. Stated another way, this means those 7 IC are being exclusively used for a period of 95 days. Thus, the real IC cost for the 1936 Infantry division is actually (7*95) 665 total IC.