I just ordered 6 CT shirts last night, 3 of which being the tailored fit white poplin. Post some fit pics and what size you ordered if possible!
And to answer your question poplin refers to:
strong fabric produced by the rib variation of the plain weave and characterized by fine, closely spaced, crosswise ribs. It is made with heavier filling yarns and a greater number of warp yarns and is similar to broadcloth, which has even finer, more closely spaced ribs.