Here is the fundamental concepts behind Einstein's Theory of Relativity. 

• Energy and mass are two forms of the same thing, Energy = Mass times Speed of Light² 
  Or more commonly known as.. E=MC²
• The mass of any object curves space and time. The more massive an object is, the more it curves the space and time around it.
• Gravity is a result of curved space and time. So, the more massive an object is, the more gravity it has.
• Light is made up of photons which act like both a wave and a particle (dual nature).
• Light has no resting mass or weight so nothing can travel faster than light.
• The speed of light is constant (never changes) and travels in vacuo (empty space) at: 

186,000 miles or 300,000 km per second.
• Light is affected by gravity because it has momentum and energy.