Iron is metal resource from vanilla Minecraft, and an ingredient in nearly every recipe in IC² in one form or another.
These forms of iron are seen during ore processing:
These forms of iron are seen during metal shaping:
Pretty much everything ever.
No, seriously. The first thing you'll need iron for is crafting the Forge Hammer to make metal plates, in order to craft an Iron Furnace or Basic Machine Casing for your first Generator - there are very few things that don't use iron in IC², and most of them are subcomponents of things that do! Or maybe they're steel instead, which is just an upgrade of iron anyway.
You're going to need lots of this stuff. We mean it.