Generally speaking, a differential signal is one in which one signal is carried as two voltage levels on two different lines, one is positive and other negative, but with same abs vlaue wrt ground.
The advantage is that should these voltages be disturbed, the disturbance can be eliminated at the reciever by just subtraction. (Assumes that both lines are disturbed in more or less the same fashion)
It helps if you ask a specific question instead of saying just "not clear".
Say you want to send a binary one. In a diffrential encoding scheme you could send a +5V on one line and -5V on the other line, A zero would be send as -5 on the first line and +5 on the other.