That's the intertank-the cylinder that connects the bottom of the LO2 tank to the top of the LH2 tank.

[enter link description here][1]

It doesn't contain propellant, so is built for lightness and strength, with skin-stringer construction. The ribs are the stringers.


  [1]: https://science.ksc.nasa.gov/shuttle/technology/images/et-intertank_1.jpg