In simple words, Path difference is the difference in length of two paths. When two waves are travelling with same velocity they will have constant phase difference. Its just like when two car moving with the same velocity have same difference in gap between them.
The state of motion of a particle at a given place and time is called phase. The difference of phase between two particles is called phase difference. When two light waves passes through a medium, the particles of the medium vibrate. When the particle execute simple harmonic motion (complete one two or fro motion), then the wave move forward by the distance equal to its wavelength. For a complete wave, the wavelength of varies in λ and the phase is changed through 2π.
If there are two waves with the path difference of λ. Then the phase difference between them will be 2π.
If the path difference is x then phase difference is: 2 π/ λ × x.
So, phase difference = 2 π/ λ × path difference