Hello Aishwarya, 

Electric field is a vector charge surrounding an electric charge that exerts force on other charges attracting or repelling them.

Magnetic field is a vector field that describes the magnetic influence of electric currents and magnetized materials

Electromagnetic waves are a result of interference between electric field and magnetic field. So when electric field magnetic field waves are not in phase there will be destructive interference between them because of which there will be reduction in the amplitude of the wave and subsequently intensity will be reduced as intensity varies directly with the square of the amplitude. 

Hello Aishwaryaa,

The electromagnetic waves form an important part of the Physics syllabus of the class 12th examination and a lot of questions are asked on them, so do study them well. 

As for your question, electromagnetic waves are a result of interference between electric and magnetic field waves. If in case they are out of phase, then there would be destructive interference between them which will cause a reduction in the amplitude of the wave and subsequently, the intensity will also be reduced as intensity varies directly with the square of amplitude.