I have heard that it represents the rivers Nile and Euphrates......Do Jews actually believe that all that land is theirs as supposedly "promised" by God?
Wherever you heard it, please do yourself a favor and stop reading biased propagandic websites which spread lies. You'd be so much better off without them.
The Israeli flag is compromised of 2 Jewish symbols. Magen David, also known as "The star of David", One of Israel's most famous kings in the bible, the one who fought against Golliath. The 2 lines represented the "Tallit", a prayer shawd used by Jews. Here's a picture:
http://www.kodeshop.com/images/tallithtsemer.jpg