I know am sounding dumb but i have not been able to find a correct definition of glare
What are the reasons it props up into images For example am taking a photograph of a fruit in diffuse light :
What is this glare due to and how do i avoid it?
Glare as you put it is more formally known as a specular reflection. This occurs when you have smooth objects where all the light bounces off in the same direction. This contrasts with rough objects which due to fragments of the surface pointing in different directions, reflects light in different directions.
Most objects like your apple produce both specular and diffuse reflections. Specular reflected light is often partially polarized in the process so you can reduce it with a polarizing filter. Alternatively you can arrange your lights so that the reflected light doesn't hit the camera.
Highlights and shadows are what make the subject appear real and lifelike, and to some extent, glare is part of that. It's just a matter of making the glare appear minimally or positioned best to describe your subject.
Anyway, the best source to learn about controlling glare, how it happens, and all sort of other lighting information: Light: Science & Magic