TextureCooridnate node describes the 2D coordinates used to map the image to the 3D coordinate system of the IndexedFaceSet. We are mostly firmilar with the X and Y axis, but here we have the S and T axis. These correspond directly with the X and Y axis.
The coordinate system for S and T range from 0 to 1. So if you have a 128 pixel wide by 320 pixel high, your coordinate system for S is 0/128 to 128/128 and for T is 0/320 to 320/320.