Physics (stationary wave)

For a travelling wave, the oscillation of each particle, in respect to frequency and amplifude, is identical. They only differ in time. Hence, "in phase" means two particles having the same velocity (both in magnitude and direction) and displacement at ANY instant of time.

However, for a stationary wave, the oscillation of each particle is NOT identical. They differ in amplitude. Here, "in phase" means two particles reach their maximum and minimum velocity and displacements at the same time.

Stationary wave is formed by the interference of a wave and the wave formed by the reflection of the wave. The wave is 'fixed' at some places known as nodes.
Consider the loop at its optimum positions. At that instant, all the particles on the loop have reached their maximum displacement, are momentarily at rest, and will start moving towards the equilibrium position in the next moment. Therefore, the particles in the same loop are considered to be vibrating in phase, although their maximum displacements are different.

"in phase" means that particles are travelling in the same direction at that instant

Which means all particles between two nodes have the same period but different amplitude