Part of the Satisfactory beginner walkthrough. Series overview.
Basic Steel Production is the Tier 3 milestone the rest of the tier depends on. Steel is the first material that needs both iron and coal, and it comes out of a brand-new machine. It also unlocks a lot of the structural building, and one explosive.
The Foundry makes Steel Ingots by melting Iron Ore and Coal together. Unlike the Smelter, which takes one solid input, the Foundry takes two, so the same Coal you set up for Coal Power now also feeds steel. From those ingots you get Steel Beam and Steel Pipe, the parts a lot of bigger structures and machines are built from.
The milestone also unlocks the Versatile Framework, which is Project Part #2 for the Space Elevator, so you can start stockpiling it toward the next delivery phase. Rounding it out is a batch of New Shop Products for the AWESOME Shop. The whole thing costs 50 Modular Frame, 150 Rotor, 500 Concrete, and 1,000 Wire.
Steel Pipe also unlocks Nobelisk, the explosive you need to blow the rock caps off resource nodes. Until steel is flowing, those capped Caterium and Quartz nodes stay stuck on Portable Miners. Take this milestone as soon as the Coal is running.
Keep going: the walkthrough overview lists every part of the series in order.