Hi i have read that one signal is polarized when the phase remains the same (the phase difference is always the same) But what is the phase difference?
In what context are you referring? The phase differernce of a periodic function is simply the time difference between corresponding points of two different functions. For example, three phase electrical. The phase difference is 120°. In vibrations, there are phase differences as well.
Take a look here: http://www.du.edu/~jcalvert/tech/threeph.htm
and here: http://cipco.apogee.net/foe/frwt.asp