WHAT IS PHASE? i didnt understand it completely plz explain.............

Phase is basically the angular displacement of a particle.

For example,

y = A sin(ωt - kx + Φ)

The quantity (ωt - kx + Φ) is called the phase of the wave. The phase determines the displacement of the wave at any position and at any instant.

 

To explain this shortly, phase is the period after which the wave crosses the zero mark in its motion.

 

Example: A sine wave starts at zero at time t = 0. Then after t = π/2, the wave will have amplitude as zero because sin π/2 = 0.