i just read this article:
it says that in order to prevent crosstalk between two signal layer they must be orthogonal....
don't know the meaning of that.
crossing the lines in 90 degrees? or just routing the signal over a large conductive plane in a way that a signal1 will be moved horizontally while signal2 will be moved verticly?
please explain 
The lines in 2 adjacent planes must be routed in 90 degrees. If in the first plane you route the lines vertically, in the second you must route the lines horizontally. In that way the lines (and the signals) will not travel in long ways parallel an the signals will not "mix".
But why are you bothered about cross talk anyway? It is normally a secondary effect only important in analogue signals.
crossing the lines in 90 degrees?
Yes that is what orthogonal means.