Question

what is light metre ​

Answer 1

LIGHT METRE :-

A 'light meter' is a handheld device that used measure or 'meter' light.

A light meter can read the ambient light in a scene, or the direct light from a light source and calculate the correct shutter speed and aperture values required to capture an accurate exposure.

Answer 2

It's a device that is supposed to detect the presence of visible electromagnetic radiations from a given source and predict it's speed.