It refers to the type of yarn used to weave the cloth; nothing to do with the cut of the finished garment . High twist yarn is harder and stronger than "low twist": fresco is a high twist cloth, flannel a low twist. As i understand it, it has to do with the angle and tightness with which the separate strands of wool are woven together to make the yarn.
I suggest you lurk on the forum for a while longer before you buy anything...
Affects the texture and how warm the suit will wear too. A low twist yarn, like you would find in a flannel or a tweed, will be fuzzier, trap heat better, and be warmer. Higher twist yarns will allow you to weave something like fresco, which has fairly wide spacing between threads, allowing it to breathe. Try that with a low twist yarn, and you wind up with the durability of tissue paper, though it would probably be very soft and comfy to wear.
Perfectly fine. It'll still be a solid navy suit, the fabric will just have a little depth to it from the basketweave.