Question

Meaning of the phrase " soldiers GO OUT OF STEP while crossing bridges"

Answer 1

Dear student
The soldiers never go in step due to the following reason.
Structures like bridges and buildings, although they appear to be solid and immovable, have a natural frequency of vibration within them. A force that's applied to an object at the same frequency as the object's natural frequency will amplify the vibration of the object in an occurrence called mechanical resonance.

If soldiers march in unison across the structure, they apply a force at the frequency of their step.

If their frequency is closely matched to the bridge's frequency, the soldiers' rhythmic marching will amplify the vibrational frequency of the bridge. If the mechanical resonance is strong enough, the bridge can vibrate until it collapses from the movement.
Regards