Here is an oscillating ball.

Its motion can be described as follows:

- First the ball is at v = 0 at the right
- Then it moves with v < 0 through the centre to the left
- Then it is at v = 0 at the left
- Then it moves with v > 0 through the centre to the right
- Then it repeats...

Mathematically, the phase is the "w t" in:

- x(t) = A cos (w t)