They will fly over if they want to. It's meant more as a predator barrier than for containing chickens per se, although if the chickens are happy where they are they will not necessarily bother to fly over it. (Or you could clip wings) (And of course meat birds ain't goin' anywhere).
If you don't electrify the fence, there is really no point at all in buying it (you could use a roll of cheap flimsy garden netting just as well to contain chickens who are not sufficiently motivated to fly over, and without electricity it will do zilch against predators).
Premier makes EXCELLENT quality stuff, and does EXCELLENT customer service and advice, I very highly recommend them for electric fence supplies (unless you want a type of charger that they don't carry, as their line of chargers is rather limited). For my horse fences, I find their stuff to work noticeably better than most farm store tape or rope.
I have a half roll (82') of their 4' poultry net in my garage but have not used it yet.
For more info, use the 'search' button above, there are a number of threads here on electronet in general and Premier in particular.
Pat