The "Flex-Pipe" is, as was stated above, the segment after the cat, before the "Catback"(As people call the rest of the exhaust...).
It's called a Flex-Pipe, because it has a flexible section in the middle.
This is to allow for the movement of the engine.
W/ a little "Give", the flex-pipe prevents your entire exhaust system from being torqued around every time you rev.
It goes over the front suspension's cross-pieces, etc., so it's the curved segment that's hard to see, just behind the cat.