What is simultaneity in special relativity?

In special relativity, what is an inertial frame? According to Einstein's theory, what happens to time as an object approaches the speed of light? What is time dilation? How does special relativity explain why nothing can travel faster than light? What is simultaneity in special relativity? In special relativity, what happens during collisions at relativistic speeds? If two observers are moving relative to each other, what do they see regarding each other's clocks? What role does simultaneity play in understanding events in different frames? What is the speed of light in a vacuum? What is the principle of relativity? What is length contraction? What is the Lorentz factor? What happens to an observer's perception of distance as they approach light speed? What happens to an object's mass as it approaches the speed of light? How does gravity affect light according to general relativity? What does "rest mass" refer to? What does E=mc? represent? How does special relativity affect our understanding of space and time?