What is the meaning of alines moving up in position/time graph

What is the earliest point in the universe that we can observe? why can’t we see back further than this?

Which of the following correctly describes what happens when an atomic bomb explodes? small pieces of fissionable material are joined and form a body with a mass greater than the critical mass, the relative number of neutrons escaping decreases, and a chain reaction and explosion result. large pieces of fissionable matter are brought together quickly and form a body with a mass smaller than the critical mass, the relative number of escaping neutrons increases, and a chain reaction and explosion result.

Aball horizontally thrown from a cliff at 8m/s