What do you mean by einstein famous equation E=mc²?

Albert Einstein the most famous scientist of our world popularly known for their equation E=mc²
In their popular paper spacial theory of relativity
He proved that time and distance observation dependent because velocity of light is not

Speed of light is constant for every observer indipendent of state of observer ,observer is at rest or motion doesn't affect the speed of light

So what that mean?

1) .What is observer?

A observer Can be a point anything on space .
Who can observe motion or rest of a praticle and observer also hold motion or rest state with respect to anathor observer but always at rest for its own observation .

interducing frame of referance

If two observer observing each other at rest state i mean if both observe rest or zero velocity they are in a frame of referance .

But if they observe motion of each other they are not in a frame of referance .

While a frame can be consist of many observer
Condition for single framed is every point is at zero speed for a observer in a frame while haveing f(v) constant velocity function for anathor observer in a different frame .

You can tell it

A frame-of-referance is a closed or openned system in which every thing in a constant velocity at a instant time with respect to any observer

How does speed of light affect ?

Speed of light doesn't affect observer does

You have to consider like this if there is a light in
two different frame observe same speed of light how could it possible only possible when they have different value of time and length ,

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But velocity related to momentum thus mass increases it also related kinetic energy which lets mass and energy inter convertable
Mean E=mc²